Here are the correct values, I traded in my 360 myself -
360 w/ wireless controller - $190 ($180 with a wired controller)
20 GB Hard Drive - $25
I traded mine in because MS sent me a shitty system back as a refurb that was on its way out, and I'm not playing the return / refurb game with them - I was able to combine my Edge + another 10% coupon flying around at the time, so the 360 + hard drive + some random accessories I had got me about $300 in credit.
I would have barely made $300 on Ebay after fees, and then worry about if it's going to break on a buyer, and get caught up in a Paypal dispute - that's not fun.
Now the 360 is EB's problem, not mine. It's probably already red ringed for some unfortunate sap.