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.