you may know all this already, but:
I would guess you should look to pay 600 for a usgi in decent condition, and up from there. the fewer made by a particular manufacturer the more they cost. inlands are cheaper than rock-ola's for example.
as far as the commercial guns go, avoid universal. plainfields should run 425 or so. iver johnson's are hit and miss (so I hear) and should run 375-ish.
they're a great fun gun, and capable enough with a soft or hollow point bullet. think .357, not 30-06. IMHO they are overpriced. if they drop by half or to where they were 8 years ago I will buy another.