Volume is key here, and I think people are forgetting how manipulable this factor is.

What about setting up a futures market? Even, one which is not completely closed? According to various sources (I cannot speak to their academic merit here, but I will cite one popular example: http://www.cnbc.com/id/102456187), crypto-currency's volatility in the real world is largely due to lack of a futures/options market. You could easily set this up in such a way that it allows limited designer control of prices...simply have some sort of partially-NPC driven financial sector. For instance, you could keep a library of historic prices of commodities after various types of world events, similar to what might occur in game, and simulate random, quick volatility followed by a correction to equilibrium. You wouldn't have to get the complicated stochastic calculus exactly right, as long as the equilibrium price is somewhat sensible and there exists a reasonable system ingame by which people who are crazy about that sort of thing can gamble on the market and make a profit sometimes.

This could be particularly interesting if you decide to go all out...you could have users buying up houses near the exchanges and rushing to work every day so they can beat the market. You could very well also make a whole elaborate NYSE-style trading floor. And then what if someone tries to attack the physical exchange? What if there is a stock market crash? Now that would be some MMO.