Large Coin
Vendor Coins are large floating gold coins that can be found anywhere, in both normal floors and in secret areas. Each coin gives a permanent multiplicative rebate of 0.5% to all shop prices. In other words, each coin reduces all costs by 0.5% of what they were before that one coin. That means the more of them you get, the less effective they are individually.

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:
$ y=1-.995^x $

Conversely, to know how many coins are needed to get a given reduction, the formula is:
$ x=\log_{.995} (1-y)=\frac{\log_{10} (1-y)}{\log_{10} (.995)} $