I was tossing around ideas for this also. Now, bear with me, this is very simple-minded, and really only useful if you want fairly simple code.

Basically, upon server creation, a set amount of money is made. This all depends on how many players are allowed into the game max. Also, you could scale the amount of available money up as people join. With a set amount of money, bartering becomes more evident, which I always liked.

This system could create one problem, as each person isn''t limited to the amount of money they can have, but the world is limited. Basically, hoarding comes into play. To counteract this, invoke taxes. Quite simply, scale them higher as they get richer.

It could cause problems, but it''s SOMETHING.

If you were to accomplish this I would be extremely impressed. It has been tried before. So keep up the good work.

The basic idea I have is that each item will have a certain value to them, a basic percentage that indicates it''s worth in the entire economy. I don''t think I will make this a variable, instead, I think it should stay constant. This value is not the actual price, not yet at least. The higher the value, the higher the price. Next, there is a certain supply of each item, but right now I have no idea how I will manage this aspect. The supply of each item IS a variable, it should go down when people buy things, and it should go up when things are made (time based?). Maybe I will have a function that will increase supply based on demand (sorry, still trying to figure this out). As supply goes up, price goes down (in an attempt to increase demand for the item and sell more of it). Next, each item has a demand value. I think I will manage this randomly if the game stays single-player, randomly simulating different transactions over the course of the game, with a bell curve across the value of items (mid value items get most demand, high and low value get least). If the game somehow goes MMO (I don''t know how it would do that, it wouldn''t happen as any fault of mine, hehe), then the demand values could be an accurate representation of the demand. Higher demand means higher prices as people compete over who gets the limited supply of items. This ends the attributes that are item specific. Now, we have total money (all the money in the world) and money supply (all the money in circulation, very different from total money), and money demand. Money supply would change from people hording money and taking it out of circulation or the "government" printing money. More supply means higher prices because there is more money to go around for each item. Money demand would be the sum of all the money needed to satisfy all the demand for each item. Money demand would not affect prices directly, instead it would affect the supply of money in circulation. If money demand outstrips money supply, then more money would need to be created, increasing the total money. If the insufficient availability of funds was due to one or a few people holding the money out of circulation, then when they do put the money back on the market this will increase the money supply, devalue the currency, and make prices go up (because there is more money to go around for each item). This would be inflation. I''m not sure about deflation yet. I think that if a significant amount of money is not used in a certain amount of time, then the total money would decrease...somehow.
Okay, summary:
Item value- a percentage value that each item has, commanding a certain amount of money based on supply and demand.
Item supply- a value that represents how much of each item there is to sell. Lower supply means higher prices because there is less of each item to go around, and people will compete for the limited supply. higher supply means lower prices for just the opposite reason, there is too much of each item and the prices need to be lowered in order to make people want to buy each item.
Item demand- a value that represents how much people want each item. Higer demand means higher prices, lower demand means lower prices, for much the same reasons as with supply.

Total Money- the total money in the world, increasing over time (generally) to meet the demand. higher total money would not necessarily affect prices.
Money Supply- all the money currently in use and available for any transaction. If one player has a lot of money, then that money is out of circulation and cannot be used by other players for their transactions. An increase in money supply will mean an increase in prices because there is more money to be spent on the same amount of items.
Money Demand- all the money needed to satisfy all the item demand. This will not directly affect prices, but if the demand outstrips the supply, more money will need to be added to the total. Money cannot be directly added to the supply without item supply being reduced.

So now, we should be able to come up with a function describing all this...let me think for a minute
price=(item value*item demand*money supply)/item supply)
item value=constant, preset
item demand=variable, based on player transactions (how that will be done, look under a rock for a better answer than the one I got)
item supply=variable, based on item demand
money supply=variable, based on transactions made

And this isn''t even getting into taxes and on what the tax money will be spent.