I think one issue here is that minerals and actual currency are separate things. Anyway, one way to combat inflation would be to limit currency generation - in a real economy currency is created at a fixed rate, not when people earn it. Essentially, when you kill an NPC and steal their wallet you would be taking from the "pool" of NPC money. As the NPC money pool decreases, you get less and less money from killing NPCs and vice versa. This could inherently push the game towards PVP as it evolves.

As for "drops" and various bits of incredible loot, the same system can be applied. If you make most rewards based on increasing your characters skills or something, you can limit the amount of +7 sword of bats without causing too much of a problem.