According to S&W and most other old school rules there are 10 coins/pound regardless of type.
How much is that in real world terms? Roughly 8 modern US quarters.
The problem with this becomes more apparent when you consider older (1950s) US quarters which were silver. It took about 7.25 of them to equal one silver piece. Pennies, actual copper pennies, run a little over half that weight implying 12-13 pennies per copper piece.
I think I’m going to run with items roughly the weight of a silver quarter being a coin in game terms. The ratio would be about 116 coins per pound. Just to make life easy I’ll probably go with either 100 coins/pound or 96 coins/pound. I realize this creates issue with the use of encumbrance to make players be choosy on treasure, but I’m doubtful this will be as damaging as it might seem.