I agree with you, there is no excellent way to value. that's where the common sense comes in. I just passed on a 68 numbers matching GTS cause I just could not place value on it, couldn't find someone to verify it and my common sense said no to his asking price. It also didn't help that I am not well versed in GTS's or numbers matching 68 cars.
do I have regret? YES it feels like it slipped through my fingers, but common sense said no.