The AD&D system at first seems really clunky:
200 cp = 20 sp = 2 ep = 1 gp = 1/5 pp
At first, I seriously thought about "house-ruling" it, going to a more easily understood decimal system, such as 100 cp = 10 sp = 1 gp = 1/10 pp
But then I realized my mistake. I was thinking of the gold piece as the base monetary unit. Can't really be blamed for that, I suppose, since EVERYTHING is priced in gp as the base unit.
In reality, the silver piece was intended as the unit. Think of it like silver dollars.
The silver piece is like 1 dollar.
The gold piece is like 20 dollars.
The platinum piece is like 100 dollars.
I guess the copper piece is like a dime. I would prefer 100 cp = 1 sp to make them like pennies (house rules calling after all, I guess...).
But anyway it makes more sense to me to think of it like that, since we naturally use dollars, 20's, and 100's as our basic currency.
The whole system is more like the US dollar before WW2, before inflation took hold. Back when most transactions were under a dollar, $20 was a big deal, and $100 was a huge transaction.
AD&D is like that. Most daily transactions take place in copper and silver. Poor people would rarely see gold. The middle class would regularly use gold in their contracted interactions. The upper classes would use gold for jewelry and status displays, and to pay their skilled workers, but they would use platinum for their big transcactions.