I'd definitely suggest an auction site. I got my wii off ebay for 70 bucks less than asking price, and then I got a 1 year warranty for 8 bucks from a reputable site.
Actually, for the 360 I wouldn't suggest that, cause they're pretty fragile. The wii is the sturdiest of the three. Anyways, my first 360 broke a week after I got it, because I moved it while a disk was inside and it moved and messed up something. XP For that I would definitely recommend a warranty. XP