“Your reward for rescuing the Duke’s daughter is a magical weapon, a suit of enchanted armor and a favour.”
As D&D characters become more and more powerful they always seem to become filthy rich along the way. At first they reinvest their newly gotten gains in themselves, purchasing new weapons, armor and magical items. But after a few levels they have everything they need and they start accumulating wealth. Massive amounts of wealth.
I have numerous characters at or above level 10 and all of them have thousands of gold pieces recorded on their character sheet. And the money continues piling up as they continue adventuring. Now I can spend the cash for the sake of spending the cash, but honestly there’s nothing that these character want or need. They already have magic items in every item slot of their character sheet. Eventually these PCs hit a point where any monetary rewards become inconsequential. I mean what can you buy with 20,000 gp that you couldn’t buy with 15,000 gp?