GameStop might get in more used ones now that they have that trade-in promotion giving $80 credit for them towards an X360. It'll probably still cost you over $100 there. If you know your way around an Xbox, you can buy "broken" ones off eBay for less than $50.
I was thinking of trying to get a really cheap one to use for the trade in deal..$319 for a Premium isn't bad at all...combined with the 3 for 10 it could be pretty easy to turn an X-Box lot into a 360 PREMIUM