When I moved recently, I decided to make the jump to an HTPC setup. Therefore, I was suddenly in the market for 3-4 Xbox 360's to act at Media Center extenders.
The going rate for a used Xbox at a retail store was around $179 at the time. However, searching in my local area on Craigslist, there were plenty listed in the $70 range, and I was able to negotiate down to an average price of $40 per Xbox 360.
Unless the Xbox 360 were a gift, you could save a ton of money going this route. I was even able to make sellers show me the Xbox plugged in, working, and able to login to Xbox live.
I did observe that most of the folks that were selling their Xbox 360's for pennies on the dollar were folks that probably shouldn't have bought them in the first place. In at least one case, I didn't drive nearly as hard of a bargain as I could have.