I "fixed" this in my campaign setting by spending an inordinate amount of time translating local goods into trade currencies like goats, gold, and gems, and then translating those into local currencies, which are sometimes equivalent to standard currencies.
Oftentimes, I tell the players that they are not really picking up 50 gp, they are picking up the equivalent of 50 gp in the local currency system. So, it's like fantasy Bitcoin that everyone can get.
I've experimented with changing exchange rates, but, generally, the players assume that they've traded their "equivalent of 50 gp" for trade goods like gold, copper, silver, and sheep, and then sold those to earn local currency. It's much simpler that way, and all of my regions can have different currencies.
As for the wealth gap, I've "fixed" this by adjusting the default pay rates of laborers and craftspeople, coupled with giving PCs slightly less loot.
In the Egypt of long ago, laborers were paid in clothing, onions, and animals.
In my worlds, similar pay schemes often occur.
A laborer might make 3sp per day (enough to buy two poor meals, with one sp left over to be saved). The tax collector takes 30% of that 1 sp, leaving the laborer with 7cp per day, not counting meals. Two sp per day is listed as the rate of "poor" living, and the 1sp (-3cp) extra allows for a more reasonable income rate.
I have constructed economic tables, full of Deltas and other economic limits and such, but, really, it's as simple as doubling or tripling default DMG incomes, and factoring in a few taxes.