There are a variety of factors, similar to those determining the actual store price of the item. Of course, there's the age of the game, the supply and demand of the game, and how good the game is overall. Because of this system, newer games are more expensive, but are also traded for more. In cases of extreme damage, the trade-in value of the game can be lowered, like if the disc had a very deep scratch or the case was shattered. Generally, if you compare the store prices of the game to the values given by trading that game in, the game is usually 1/3 the price of the new game.