I would say use Edmunds True Market Value appraisal system, find the Trade-in Price, and take at least 30% off that number. Also check with your insurance company as some companies will not insure salvaged vehicles; also any extended warranties are void. Also you may not be able to get financing.
Keep in mind that whatever gets you in cheap on a car will come back to haunt you when you sell; so buying a salvage car isn't necessarily a bad idea but it's a bad idea to pay anything but a really REALLY bargain basement price for it.
Here's your linkn to True Market Value: http://www.edmunds.com/tmv/used/index.h