I think that's something that hasn't been fully "solved" in any MMO. Money gets created out of nothing and, with enough time, prices inflate into absurdity. Even with auction house taxes and other sinks that forces players to spend their money, they still end up creating more money than is destroyed.

Trying to implement a wealth tax system would be hard, as players could easily find loopholes in order to avoid paying most or all of it, whether by using alts or keeping most of the money as things that wouldn't enter the math for their "net worth". Not to mention that, depending on the setting, it would create a massive shitstorm.