lol if u get from ebay or somewhere, even a used/re-certified one..........im like 75% sure it's gonna fail on you and get the RROD. Then if you still even have a warranty, which the xbox probably won't, you can send it back to Microsoft which will take maybe a month.
Get a new one if you can.........you risk a lot when getting a used Xbox 360, especially the fact that like 80% of them got RROD's when they were first made. Of course microsoft made revisions each year about with new processors and such and the malfunction rate decreased but you won't be able to know which chip you got if it's an old one.
If the Xbox 360 is 1 year old, you should be fine, they will most likely have updated chips. Mostly anything older than that will have older revisions that are bad and still gave RROD's. They started to implement this new chip with the Elite's when they first came out that were supposed to be better......thing is, some and a good amount still got RROD's. My friend got it on his Elite when it first came out.
Im just saying, be careful of which you buy. New ones will be more safe.
Anyways, cheapest place........look around, there are deals all the time on new Xbox's. I think Newegg had like an Xbox 360 Elite bundle for $250, its probably gone by now.
wallmart.com has some good xbox 360 bundles and deals, most wholesale club stores do too like Costco. Wallmart.com has the new FINAL FANTASY EDITION XBOX!!! too bad its nothing special, just comes with the game, 2 controllers, and a 250gb which isn't a bad deal. You probably don't need 250gb and the 120gb are much cheaper with great bundles.