Don’t even need to go that far if you’re a wizard or cleric. It’s not totally infinite but selling your spell slots for the day is a thing in almost every world I’ve played in, and those fuckers are expensive! 10gp per spell per spell level seems to be the going rate.
You don’t have to guess the value of gold pieces. It stated clearly in 2nd and 3rd edition that coins of any type are 1/10 of an oz of the substance.