example, I bought 2 copies of Silent Scope 3 on PS2. They give $10 each. Well, I traded in 4 games, and when you trade more than 3 you get $5 extra on each, making Silent Scope $15 - thus for the 2 silent scopes, i have $30.
Next, if you preorder a game and trade in two, you get it for $30.
$30 credit for the games - $30 for a preorder = $10 out of pocket.