Other online games did this by fiat. Gold pieces were reposited in dungeons guarded by monsters. The more powerful the monster, the more gold it was squatting on. To get the gold, you had to kill the monster, and building a character powerful enough to do so required a certain amount of time and effort. The system functioned okay, but in the end, the decision as to where the gold was located and how much effort was needed to win it was just an arbitrary choice made by a geek in a cubicle somewhere.