If you have been missing on the game since November, then there is no reason to buy the game higher than $30. Once COD Worlds at War comes out, COD4 will drop to $30 or less. It's already qualified for the Platinum Hits with its stellar sales number.
ideally I'd want it for $25 and no more than $30, but I doubt that will happen. I don't even like the game, but I have a friend that wants to play online with it...