I guarantee you that you will not see this game decrease in price. At least not from any mainstream store. It will take a loong time. ODST Dropped in price due to it not selling that well. I noticed this when I bought it about 2 weeks after it was released.
A better question is.. how do they sometimes manage to have a used copy so quickly? Within days.. Parents find out?? LOL
then probably Newegg offering $5 off MW2 for Vetern’s Day would best so far?