For a like new condition, anything under $500 is a steal! Definitely buy it - and if you don't like it, I'll buy it off you for $475 - so you can make a quick $50 if you don't like it (I'm serious - email me if you want).
Pre or post doesn't matter that much - since the only difference is the flash supressor that the pre-ban has, but the pre-bans sometimes have a few bugs that the post-bans don't have. Don't worry if it's pre-ban - it's worth a little more, and Bushmaster will gladly help with the bugs. Either way, it's a great deal.
I bought one a little over half a year ago for $400 (like new condition as well) and it was the best gun bargain I have EVER made. It was for sale on this board, and some people still hate me for having been the lucky guy that got it. [:D]
Keep in mind it will eventually cost you a little more, since you WILL need optics for it (the fixed sights suck) and you might want to have some modifications made by a gunsmit at some point.