Usually they are the price of keys (2.50 USD) compaired to the steam store price.

Lets say Fallout 3 is the case here.

It's 20 USD, meaning it should be around 8 keys.

Just take the price of the game divided by the price of one key.
 
Usually they are the price of keys (2.50 USD) compaired to the steam store price.

Lets say Fallout 3 is the case here.

It's 20 USD, meaning it should be around 8 keys.

Just take the price of the game divided by the price of one key.
but keys aren't 2.50. And thats wrong anyways, dem sales.