The overall price reduction is displayed at the bottom right corner when talking to a vendor. The value displayed is rounded to one decimal place, but the prices actually use exact reduction values. Final prices are then rounded down to the nearest integer.
If "x" is the total amount of coins obtained and "y" is the overall reduction, the formula to obtain "y" from a given "x" is:
Conversely, to know how many coins are needed to get a given reduction, the formula is: