I can't find where this was (I keep thinking it was in Mongoose's Conan d20 game, but I'm not able to confirm that), but I distinctly recall seeing an RPG where you received XP for GP, but only for GP that you
spent. Furthermore, it had to be spent on inconsequential things, rather than stuff that had game mechanics.
So for example, if you spent 15 gp on a new longsword, then you gained 0 XP. But if you spent 50 gp on
ale and whores, you gained 50 XP for that. The idea was that this kept driving the PCs to go adventuring for gold (rather than murderhoboing) and then immediately spend what they recovered so that they'd have to keep doing it. It was a great mechanic.