Another idea. Figure out your max expected population for the world, and how much money would be reasonable for that population. Put that much money in the world, but have most of it be mineral that must be mined up and converted into coins.

Allow humanoid NPCs to set up mining and don't give out money for non-humanoid npcs when it doesn't make sense for them to have money anyway. You can allow a skinning system to get material out of npcs that don't give money that they could trade for money(doesn't have to be just skins, claws, teeth, poison glands, whatever valuable body part the animal might have).

If player growth outpaces NPC mining's generation of money currency will deflate and encourage players to set up their own mines(gold rush?) until you eventually reach your theoretical max and things get balanced out. When players close their accounts you could convert the money they had back into mineral wealth in a new vein or extending an old vein.