I got a premium 360 for Christmas, and it was purchased for 300. It was a $100 rebate through the store Microcenter. I think that deal is long gone, however, so you'll have to wait until another similar deal is offered at some retail outlet.
The current price tag for the premium edition is of course 400, and will likely remain that way for a long while, since there is no need to drop it with the premium PS3 being 200 more at 600.
I'd check out Craigslist, I've managed to buy quite a few electronic things through there, and if you keep your eyes open and avoid the "too good to be true" posts, you'll probably be able to nab a used but in good condition 360 with a couple of games for under the 400 price tag.
Good luck. I'm looking forward to Guitar Hero 2 as well.