Would it be workable to provide a spinner for each coin type (plat, gold, silver, copper) such that the price of the item was reflected in the values assigned to the spinners, then let the user adjust one spinner and see the others change immediately?
For this to work, the algorithm would need to "make change" to convert larger units into smaller ones, and should similarly have an option to coalesce smaller coin units into larger ones (or "chip up", as poker players like to call it
).