gistfile1.txt

When you buy something the percentage you paid above the minimum is stored with
the purchase. When checking to see if purchase has access to a file with a
separate minimum price the percentage above is applied to the current minimum
price to calculate "effective purchase pirce". 


For example, you have a game for 5 dollars, and a bonus file at 10 dollars.

A user buys the game for 10 dollars to get the extra download.

In the future you decide to double the price to 10 and 20 dollars.

The user paid 100% more than the minimum originally, so their effective
purchase price is now 20 dollars, so they still have access to the extra file