I wonder how hard a free market would be to simulate.
Well, hard is a tough question to answer.
In terms of technical limitations, not hard at all. Very detailed world simulations are possible through Excel, ranging from climate predictions to economic projections.
In terms of design? INCREDIBLY difficult. Each formula, of which there must be hundreds - if not thousands - must be painstakingly crafted and tested to an ungodly level to not only work in a logical manner, but also to deal with an insane level of quirks that could arise, or else the entire system opens itself up to manipulation to an unnatural level by a player. For each variable (read as: good sold, vendor inventory, location/region and trade routes) and it becomes a tangled web of problems.
The most difficult part is the complete lack of ability to easily apply real world formulas to these simulations. Since a video game doesn't have direct examples to "test" their formulas out on to discover if they make sense to the real world, meaning the only way to refine is through large scale testing and experimentation every step of the way.
Not to say it is impossible, but it would be a huge endeavor that would require that the economy be a large part to the game to warrant the amount of work involved, instead of an ancillary system.