Some games sell game content. I think DDO does this, but I'm not positive. Players purchase access to new areas of the game, things like transportation fees, but very similar in practice to the expansion pack model, where you buy whole dungeons or quest packs. Many games use real cash for that, but in-game currency works too.

Any time you have a money sink of some kind, take the time to balance it. Measure the game, figure out how much it actually costs players to obtain a resource, and try to make it cost about the same amount. For a dungeon encounter, compare the average cost per player to the average value of loot obtainable.