Buying Experience

Although I personally have no problems with XP for GP, since I see XP as a measure of reputation gained from bragging about monsters slain and bringing back large sums of cash (and the resulting increase in confidence in your own abilities,) I do want to help out those who interpret XP as literal experience and practice. Yanking XP for GP will severely limit options for advancement and slow advancement down, unless adventurers just start murdering monsters in huge numbers. But how can you make XP for GP less dissociative?

Easy. Let PCs pay for training. Not in the same way it's handled in AD&D, but a straight up purchase of XP, explained in-game as training in your class. If a PC can find an NPC with a higher level in their chosen class, they can purchase training at a rate of 1 GP spent yielding 1 XP in return. Time required is 1 week per 1,000 XP (this can be adjusted to suit your preferred advancement rate.)

Less qualified teachers have a less favorable rate (3 GP per 2 XP.) No teacher at all, just self training, costs 2 GP per 1 XP. Excellent teachers with a favorable reaction can train PCs at a rate of 2 GP per 3 XP or even 1 GP per 2 XP, the highest possible rate.

Under this scheme, it doesn't matter how the PC acquires the cash. However, retrieving treasure is still the fastest way to get lots of cash, speeding up advancement.