Here in the Northeast, 350,360, and 450's are still available at a reasonable price.
The Honda 500,550, are really pricey now because of the Cafe style rage and the parts interchangeability . I have seen some 78-79 750's around for less than earlier model 750's and understand that the earlier model's top end is better than the latter years. The best priced honda 4's seem to be the 80-81 650's,lots a balls ,handles well ,easy to work on,but those years in the custom series have allot of interchangeable parts. Single cylinder cb's are making a come back and are still affordable and easy to work on.
Kawasakis have a following that pays large $ 70's KZ series,sometimes I find them for a grand in rideable shape, most times not. They've been run hard.
Yamaha's , reasonable prices,parts availability okay, but allot of what I see is bobbers, haven't seen many stock Yammie road bikes out there. Have seen lots of smaller cc 2 stroke enduro.
What type of machine are you looking for? The Philly craigslist usually has all kinds of older machines for sale...and by the way, never say classic unless you already own it, it just brings the price up.