My sense of things is that the currency of souls among the demonic is something that should strike horror into the hearts of Player Characters, and it is to that point that I'd like to address my reply, eventually. Before I do, however, I have a few thoughts for general consideration.
In the Judeo-Christian world view the Devil certainly has a desire to collect souls. What gives those souls `currency' in your conception is that they are tradable commodities like coins. One devil trades two 'medium hot' souls for one 'very hot' soul, perhaps? The question of how this currency actually works is something I'm struggling with. While I find it fascinating as a story concept, I'm not sure how I would take this approach.
A few comments on the Economics of Hell...
To my mind the souls of the damned in hell are not necessarily traded, or tradable. Are they not rather prisoners in the eternal dungeon of despair? Even the demons who rule over and punish them are themselves prisoners (remember that they started out in heaven). While Hell has its economy I'm wondering if one seriously would be able to include souls as tradable. If we look at Dante's Inferno we find that it's not to specific demons that the souls are condemned (though it may be implied, I do not find it articulated), but rather to specific punishments which are in different places among the nine circles of hell. The punishments in fact are allegorically linked to the nature of the sins committed. Here is an example for Lust:
"Second Circle. Those overcome by lust are punished in this circle. These souls are blown about to and fro by a violent storm, without hope of rest. This symbolizes the power of lust to blow one about needlessly and aimlessly. Francesca da Rimini tells Dante how she and her husband's brother Paolo committed adultery and died a violent death at the hands of her husband. (Canto V)"
- The Divine Comedy
The second circle of Hell is not said to be populated by demons, though we might suppose that demons may periodically roam the area tormenting the souls there, though perhaps not. If some greater demon is in charge of this region it is not evident from "The Inferno". The point is that no "ownership" of these souls is indicated. What we do not see is an owner of the region, and I think it raises a problem with the concept of souls-as-a-medium-of- exchange. There is no Demon of Lust, or none that is apparently in charge, so there is no one to "own" these souls, except in the general sense of the Devil "owning" them.
In fact, one even has to question that assumption. Though the Devil may (or may not) have bought them, was the transaction itself legal by the laws of the Cosmic Economy (coining new term here – Cosmic Economy is combination of the Celestial and Nocturnal Economies)? As chief Law-Breaker the Devil is undoubtedly conducting illegal activities. So the question is: Are those souls really "owned" by the Devil, even if the victim signs a contract?
This question reaches a little further back – who is the original "owner" of the soul? Is it the individual, or is it the Creator of the soul? In other words, does the Soul own itself, or is it owned by God, who creates it? Perhaps in the Celestial Economy it is God who owns the souls, though when they get corrupted by diabolical interaction (sin), and particular the "selling" of the soul to the Devil, then God casts them away – not selling them, but in effect throwing them in the garbage (Hell) as "ruined goods". We get an indication of this from the Hebrew term for Hell, Gehenna which was, if I recall correctly from prior research, a perpetually burning garbage heap outside of Jerusalem. So does the Devil really "own" the souls he purchases, or has he by his purchase simply rendered the goods garbage and useless in some larger sense? To answer this would require us to explore the question of what all of these Souls are actually created for in the long run. Does God have an intention to actually use and benefit from these souls at some future time? Is that what is meant by the allegory of harvesting souls? Well, in any event that question is beyond the scope of this discussion, though at some level it may well have a bearing on the question of the Celestial Economy, since it may well be that the Celestial Economy in fact is dealt with by these very same non-fallen Souls. As for whether or not the purchased (ruined) souls are owned by the Devil or not, according to himself he does own them, and projects that point of view to his cohorts in Hell, whether it is true or not. But according to God? My guess: Doubtful. My hunch is that God looks at the entire mass of Hell as a great garbage-can into which the refuse of ruined souls are thrown and abandoned like a host of bad apples, the Devil included. What claims are made in Hell about ownership of souls is simply irrelevant to the Celestial Economy as those "Goods" are no longer at issue having been tainted and therefore ruined by one means or another.
So within the hierarchy of Hell, might we make the case that the Devil owns everyone and everything? Well, not according to Milton's vision in "Paradise Lost" where the Demons ALMOST seem more like Peers rather than Subjects, though I think this is a false projection on the part of the Devil in the poem - he is the Tyrant, not the President of Hell, so the democratic behavior is but another ruse and illusion - fittingly enough. What I find interesting here is the contrast between the Divine Monarchy in Heaven and the Democratization of Hell. In Hell the Demons seem to negotiate with Lucifer as he makes proposals, and he must persuade them of the rightness of his decisions (which invariably are wrong). They follow him because, in essence they are stupider than he is, and so to them his plans always appear to be better than their own, and his reasoning the most "Truthful" and accurate. They also have a stake in continuing to follow him, as pride would disallow them from admitting their prior mistake. (Remember, the denizens of Hell still think they will eventually win and conquer Heaven and rule the Universe. They do not admit, or believe even, that they are in a garbage can that is eventually to be emptied into the Lake of Fire. They see themselves as the innocent and oppressed, and accuse God of unfairness of Judgement which is the basis of their rebellion.) And there is another reason that might make the demons of Hell obey the Devil – he is more Powerful than they. Thus by force of will and implied threat of force he can overwhelm them, though that power is more implied it would seem than exercised. The Devil's goal after all is to prove that God is a liar in order to justify the Rebellion (Treason), and he wants to project on Heaven all of his own sins as if he is the Righteous One and God is the Guilty One. So Hell is a crooked mirror image of Heaven in some sense – it's exact opposite.
Thus, we could compare the government of Hell with the Government of Heaven, where God who knows all things, and is unsurpassed, is obeyed without any need for question, or negotiation. His plan, simply stated, is the Best Possible, and so all who dwell in Heaven are Subjects of God, not his Peers. He is unquestionably the King of Heaven. Those who don't think so, don't belong there. So this distinction between the governments of Heaven and Hell are of interest to the discussion of the Cosmic Economics as Governments always have some bearing on Economics generally.
But let me not skip over the fact that I started out with a vision of the Economics of Olympus, not Heaven in the Judeo-Christian conception, and Heaven is not at all the same as Olympus. As you can tell, I am trying to fuse Mythologies into one grand system, which may or may not be valid, or even possible. However, that's what I currently have in mind, so to continue...
In Olympus the Gods are not the All-Knowing Creators. While Uranus (a Primordial) was King, and then Kronos (Titan), and then Zeus (Olympian) in turns, none of them are nearly as All-Powerful as the One God of the Judeo-Christian concept. While I started out the discussion with a conception of the Greek (and Norse) deities as having to have an Economy due to their limitations, this could not reasonably apply to the Judeo-Christian God who is in fact THE Creator, and thus one should question if Heaven needs an Economy in the sense I have been discussing the topic thus far. Perhaps not. After all, Heaven is, unlike Olympus, a truly Otherworldly and purely spiritual Realm.
"Dante portrays heaven as nine concentric spheres, situated between the earth and the infinity. These spheres are still spatial, but inhabited by immaterial entities, although they can still be perceived as pure forms, visions of light.
Because of the nature of the vision, such senses as taste, tact, and smell, are absent in Paradiso -- we are left only sight and hearing. Vision becomes the center of the representation.
The physical structure is taken from medieval astronomy and astrology, which was then considered science. This makes Paradiso also a scientific poem, while at the same time is a journey outside space and time.
In fact, Dante moves along without realizing these two dimensions. He goes very fast, and at the same time it seems he is not moving.
This is because the heavens are only manifestations, and not objective realities. The nine spheres are only projection into space and time of the Empyrean: the real Heaven, a dimension outside space and time."
Olympus, in contrast, seems more physical, almost as it were half way between Earth and Heaven, requiring the Gods to eat, sleep, and come for doctoring when they are wounded. Heaven seems to require none of these attributes, nor do we discover any material aspect regarding it in Dante's "Paradiso". So, the realm of Olympus, and the 12 Olympians who correspond with the Planets and Constellations have a material and Economic, aspect, but Heaven does not.
A bit more on the buying and selling of souls...
While the buying and selling of souls is something that is familiar already in numerous tales, it does not necessarily follow that from this we can derive the concept of Souls as a medium of exchange, though it is possible to create that construct in your World, my efforts are toward making my system correspond to existing mythologies wherever possible. We learn from various sources, that the Devil will gladly buy your soul from you in exchange for limited worldly advantages, material, magical or otherwise. And witches will sell their souls to the devil in exchange for worldly power for some specified period of time, at which point they must then go to Hell.
But the buying and selling in these cases is between the Devil and the humans with whom he is negotiating. However, this raises a question in my mind. Does the Devil, who bargains for the soul, then sell that soul in Hell to lesser demons? Perhaps, if this is the case, one would then ask, for what would the Devil exchange a soul to other lesser demons? Are not souls the most precious things he "owns"? Can he transfer that "ownership"? And if so, for what would other demons bargain for such souls? Or are they given out as gifts or bequeathed for loyalty or Unrighteous Deeds or favors? Or are they instead foisted upon the demons as a form of Responsibility, and not to be considered a gift or currency, but instead is a burden to them, in so far as they must spend their time torturing their souls as prison wardens spend time shuffling prisoners around the prison. It's not that prison wardens trade prisoners and gain advantage from them. It's a job, not an adventure, and the only profit they gain from it is perhaps some enjoyment at the suffering of others, or perhaps some advantages otherwise.
So I've come down to two basic splits. One is that I'm not so sure that Souls as Currency is where I would take it, though I have to say I find the idea interesting, and possibly amusing depending on how this would be done, or potentially horrifying if done with that effect in mind. Two, the inclusion of Hell (as opposed to the underworld of the Giants, Titans, and Greater Monsters of the Greek Myths), which does require an Economy (the Devil is not a Creator – and so anything that he builds must require an economy of some sort), does not imply that Heaven also must have an Economy. Heaven it seems stands above and beyond the Universe and may not require an Economy at all. So we have a Cosmic Economy that extends from the bottom of the Universe, Hell, up to the top of Olympus (or its analogs in other mythological systems such as Asgard), all of which require goods and services of a material nature, and therefore a medium of exchange. Perhaps Souls form some medium of exchange in Hell. Perhaps not. Or perhaps ONLY in Hell do they do so. And if so, then perhaps that medium of exchange is invalid and essentially false by the standards of the rest of the Cosmic Economy. After all souls do not get traded anywhere else other than Hell so far as I know, if they even get traded there at all.
Now, based on what I have so far, I have two last points. One is that the trading of souls in Hell, or to the Devil, should evoke a sense of horror in the Players. I say this because in effect this is the most horrible thing that could happen to a person – to trade their immortal soul for some temporary gain and in the end wind up eternally trapped in the prison of Hell.
The second, and even more interesting point goes to Jarod's question of whether we might contemplate that the Cosmic Economy is "the driving force behind the visible, day to day economy of the world"? This question raises all kinds of pertinent World Weaving possibilities. It very well could be, and if so, then how? It may even be the case that the Cosmic Economy is THE driving force of day- to-day world, and our players simply do not know it, nor do they ever get much of a chance to discover it. One might consider that the Nocturnal Economy is the sort of thing driven by the needs of Hell, ultimately. It might be that in another way it is driven by the imperatives of the Celestials. It may be that where these two forces meet we find conflict on a supernatural scale. The ordinary world gets disrupted, and behind that disruption is the story of two different Boardrooms, each vying for resources or some sort of Cosmic Economic advantage.
A quote comes to mind: "In war the inexperienced soldiers talk about tactics; the generals talk logistics." Economy is all about logistics. And from the logistics of the Cosmic Economy we may be able to draw forth an endless stream of stories, all of which beneath the surface make some coherent whole dimly visible to those who have a discerning eye (or happen to have read these posts, perhaps). My advocacy for keeping this material subliminal is so that the concept does not simply blurt itself out at the Players. I would say that in the long run the greater fun of Literary Worlds would be in the discovery by the Players that there actually IS more to the World than meets the eye, and that there really is *some* reason why the Goblins are invading the Black Forest in the early autumn months. They have a plan – and that plan has an economic or political reason behind it. Waaaaay waaaay waaaaay behind it.
The eventual and difficult discovery of these facts should indeed cause the Players to feel some sense of Horror. The world is indeed vast - and at the bottom of it all is something very frightening indeed: The Devil frozen up to his chest in a Lake of Ice. This feeling of horror, naturally, can then be juxtaposed to that exalted awareness of a Heavenly answer to all of this evil-beneath-the-surface-of-things. It is that juxtaposition, I feel, that creates the conditions within which RPG Stories of true literary merit have the greater chance of being formed and experienced.
As a final note I'd like to give one example of what I mean by the concept of the trading of souls as striking horror into the hearts of the Players and being Gamesmastered subtly. Lets say that in our World we do decide to go with the concept that the Demons do trade souls. The Players discover this in some Cthuluish type setting where they have encountered the old haunted mansion, had multiple adventures and encountered the ghost of little miss McFearson who died in Hamfest during a badly botched Goblin encounter. A blue vial is seen, but not handled by the players, as they watch via a magical mirror a bat-winged red devil passing a vial of blue vapors to a blue demon over a pit of fire. The exchange is for another vial with multi-coloried vapors entwining. The red devil laughs horribly. When the vial is passed to the demon-owner he mocks the contents with some dreadful accusation - it is the soul-in-the-vial's own sins that have brought him to these eternal straights. The vial is placed in a smoldering pit of searing hell-fire and the person's soul can be seen writhing in eternal agony, cursing and blaspheming as the mirror's image fades into darkness.
Something like that. I think from this example you can get the gist of the idea of how I would go about trying to present the vision of the Cosmic Economy (in this case a Soul Trade) to the Players. With subtlety, few explanations, and shrouded in indecipherable mystery. Remember - mortals are not *supposed* to know these things.
My feeling is that it lends itself to better story overall if the concepts herein are hinted at, rather than expostulated on by the Gamesmasters. Well, that's how I usually handle things in my own game, and it has lent itself to stories of greater depth and power than the more overt mode of making such elements as the Cosmic Economy obvious to the Players by some overt explanation of the nature of it all. No matter how tempting, my advice is: Be subtle, don't over do it, and keep your Players guessing! I see Cosmic Economics as a tool to help make the under-story more coherent, and lead to surface stories that have some hidden and intrinsic coherency. To my mind this is one way to make RPG Worlds more compelling, exciting and worthy of the designation as Literary Worlds.