A character can raise their Wealth Score by investing money. Maybe each level of bonus would cost 1,000 x Bonus GP. So for example, to raise your Wealth Bonus from a 3 to a 4 would cost 4,000 GP. From a 19 to a 20 would cost 20,000 GP.
So, if I were to approach this, I would get rid of the concept of GP altogether. Convert that gold and silver and gems into a generalized Treasure, and decide how you want infusions of Treasure to impact Wealth.