A thought inspired by a thread on the LinkedIn RPG group:
In classic D&D, instead of giving the PCs 1 XP per gp of treasure acquired, make it 1 XP per gp paid to a mentor for training.
Ignoring the word “experience” in “experience point” and “experience level”, I think this puts the horse back in front of the cart. You don’t search for treasure to earn “experience” to get better. You search for treasure in order to afford the training to get better.
Of course, you still get some gain for experience as well. This isn’t completely dumping experience for training.
It reminds me of the “wine, women, and song” rule from Dave Arneson’s First Fantasy Campaign in which PCs only got XP for treasure by spending it. It also recalls classic Traveller in which PCs spent money and time to improve their skills rather than through XP.
Although, once the PCs reach a certain level—¿ninth?—perhaps things should change so that they can mentor themselves.