I'm no dealer and I'm no expert, but...
IMHO, it would have to be at least 25% less than comparable clean title car. By comparable I mean that the quality of repairs would have to scrutinized in detail, I've seen some pretty poor rebuild jobs.
Why so many? Probably because so many get bent, they're worth so much, and there are too many people to count who are just out to rip somebody off or make a fast buck.
I'd only buy a salvage title car if you don't plan on reselling. Personally, if I found a great deal I'd buy one for a play/track car. Be careful and check out any prospects extremely closely.