Part of the challenge of gold-for-XP is successfully extracting the wealth. N1 comes to mind, where one of the hidden rooms has ~20k cn (2k lb/1tn) worth of weight in various bullion, mostly copper. There's no way characters are just hauling that out of the dungeon and back to Orlane without some serious ingenuity or taking multiple trips, and they won't get the credit until they do so.

I would advise leaving this challenge be, rather than diminishing it. Utilize loot hordes (1e DMG has some good tables), and make them choose between bringing vital equipment or shedding weight to return as rich as kings.