Let me start by saying that while elements of what I'm proposing here resemble the framework in Civilization IV (which I'll call C4 from here on), I am not saying that it is an option for an economic system. There are a number of problems with it - fundamentally, far too much of the game is geared towards the creation of new technologies, whereas D&D is a static technological bubble. Other problems include elements of culture, politics, religious effect on the economy and the military units/buildings as well. How large is a 'granary'? How do you calculate in D&D game terms the phrase, "stores 50% of food after growth"? No, I don't see basing a D&D economy on C4. Still, some things I can see working ... but that for another day.
I have a ground up economy that I have no intention of abandoning ... but for the moment, for those many readers who simply want something they can apply to their worlds, I'm examining a methodology for making such an application accessible.
When you start a village in C4, you're automatically supplied with 8 coin in addition to the 1 coin the village adds, plus whatever you receive from your hinterland. Obviously, in D&D there is no palace at the center of the village, so this must be discounted in determining its wealth. We can therefore regard only the village itself and its the hinterland.
Rather than thinking of it in video game terms as money that is created, let us instead view our one coin as money as it exists in a 'closed' system.
In my macro-system, I have methods for determining what the per capita income of any given region would be, dependent on where in the world the players are, but since this is inconvenient for a blog discussion, let's fall back on standard D&D ... the dreaded, sadly thought out taxation rate of 7 s.p. per person. We can extrapolate from this a tax rate of near 7% (which would be moderate for the medieval period, but what the hey, you can change the numbers anyway that works for you) in order to suppose a general income of every person of 100 s.p. per year. Given our previous post's description of a village with 500 persons, we could thus say the actual coin in our closed system is equal to 50,000 s.p., or 2,500 g.p. per coin in straight AD&D parlance. Thus, if the village produces (1) coin (in C4 terms), and the hinterland produces (1) coin, the total amount of money in our D&D village is twice the above, or 5,000 g.p.
Again, as before, if we have a location that produces (2) coin, such as an undeveloped gold mine, that's worth three times the value of (1) coin (see the previous post). A (1)-size village next to an undeveloped gold mine, or field if you like, would have a closed system monetary availability of 10,000 g.p. Imagine what a developed gold mine would produce (two to the fifth power times 2,500 = 80,000).
Don't get the impression that doubling the size of the village would double the available coin ... the coins are static just as the food is static. The number of people in the centre determines how much of the hinterland can be utilized, but it doesn't increase the production of the hinterland. Go that way and you'll quickly get into troubles with your system.
Let's imagine a village, however, with a simple plain, just as in the last example. That's two coin, or a money supply of 5,000 g.p. To this, we can now add the idea of velocity, which is the number of times money changes hands in the system, in order to give an impression of the effective money supply.
Now, any modern framework for the velocity of money is going to hardly reflect a medieval system. Money just did not change hands 20 times a year, as one of the stats in the velocity link above describes. I think we can make the following suppositions, however, regarding the movement of money in our system - not because they accurately reflect a real money system, but because they are PLAYABLE, a word I can't stress enough to impress the endless gits who cannot understand that in a game, all reality must be modified in order to achieve the desired purpose.
1) Let us assume that all money that departs the "closed" system for other locations ultimately feeds back into the same system. This isn't true, obviously ... most systems either bleed or accumulate wealth. But for game purposes, we can presume its static. Who does it hurt if it isn't?
2) All money in the system ultimately flows into the coffers of the master/mistress of the realm ONE TIME. Thus, in the system we are describing, if your players were master of the village in question, the money they would receive per year would be 5,000 g.p. Some of this would be in taxes, but some of it would be in fees, tolls, gifts, surplus sold goods, inheritances, seized land, etc. This makes a nice, healthy way to empower your players when they are trying to determine how much money they make from their land, as they can decide to forego money in favor of something else like more food or "hammers" - which we shall manage later on.
3) All money ultimately flows ONE TIME into the privileged class ... and in this case, that means any skilled guildsperson, the clergy, the local miller, passing bards and players, people who successfully gamble, prostitutes (which may not sound privileged to you, except that they're immoral, not unsuccessful), the army and guards protecting the manor, etc. Some of this would be money flowing up and eventually to the ruler, and some would be flowing down in wages, gifts, stolen goods, etc. This privileged class has to divide up this money between them.
4) All money ultimately flows ONE TIME into the whole lower class. Because of the number of people in the lower classes, this allows each very little indeed ... but for the purpose of our economy, we can suppose that they get a third of the pie (or all of the pie, one third of the time).
Given this, the velocity of our money is 3. The total GNP is thus 15,000 g.p. yearly. This is not a very meaningful statistic in your D&D world (the players will only care about how much money they get), but it's there for you if you have a way to apply it.
Let me say this very clearly: in no way should you try to use C4 for pricing items. Seriously, just don't do it. If someone wants to build a mill, or buy an army, or otherwise take advantage of their coin, buy it in the normal, D&D manner. If you haven't got a price for something, you might examine C4 for a guideline, but I wouldn't recommend it. In this case, we're only using C4 to determine the general amount of money, and NOT to provide you an equipment list. You can try it if you want, but I think you'll find very quickly that C4 is more interested in how you exchange your cash for research than for actual things, and that the actual things in the video game aren't designed for D&D practicality.
There will be more.