I have an extra Core system that I will be putting up on eBay in the next day or two. I'm banking on about $500 or so as seems to be the going rate these days.
I had sold a core for $650 cdn, but the guy has flaked on me. With the recent drop in core prices, I just handed it to my brother and told him to just pay for what I got it for. We went to Best Buy and bought some hidef cables, a wireless controller and FIFA, came up to another $200. I wish I could have grabbed another premium when I got mine, but it was the best I could do at the time.
I know several people that camped out for one (personally, the only thing I am anticipating from this, is the original Xbox will be dirt cheap). When they saw what they were going for on ebay that day (~$2500 USD), ALL of them thought it would be great to wait a few more days. Now they can only get, at best $100 more than they paid for it, and for how long they waited out in the horrible cold and wetness, it is a bit comical that they have made less per hour on selling that XBox 360 than if they would have just gone to work.
Premium ~ $750 and Core ~ $650 ... Sometimes higher, simetimes lower. But by the time you figure in fees and taxes, the margins go down a bit. Still not too bad.
