How much does a used Xbox 360 cost?

It depends on where you buy it, of course, but a used 360 from Amazon can be as low as $225 and one from eBay (because of its auctioning system) can be anywhere from $1 to $600 (though the prices can raise even more). The average price on eBay seems to be around $300. The arcade Bundle on eBay, though, seems to be quite cheap.
Also, there are stores not online, obviously, which may have used 360’s. You also should consider peripherals and games. Everything for the 360 is more expensive than even the products for Xbox and Playstation 3. Count on everything (at least if it’s a Microsoft product) to be about $10 more than the standard. I wouldn’t say this is a major deterrent, but something to consider if you’re low on money.
As for actually buying one, it just depends on what kind of games you like to play. I prefer Playstation, but if you want to play games with your friends (presumedly online), then the 360 seems to be the way to go.

Gamestop sells them like $249 used, which is not a real bad deal. Just BE SURE that when you buy it, it is the PRO system and comes with the hard drive. You cannot play online (xbox live) without the hard drive in it.

Also, check the one you buy to make sure it has an HDMI hookup, if you are going to be hooking up to an HDTV. The older 360 consoles did not have the HDMI hookup on it, and the conversion kit is like $90. Have fun, 360 is a blast!

NOW A DAYS A XBOX 360 IS 100 USED ON EBAY AT GAME STOP 169.99

i got mine for a little under $200 at a website called gameonelectronics.com they give u a brand new 360 with hd hook up a wireless controller a 20 gb hard drive and a wired headset but no games and if u go to buy games i would suggest call of duty world at war or modern warfare 2 when it hits the shelves