You can't expect them to stay cheap forever. Guys can't afford a E or B so they are going after the A, C and F's now. It will only get worse as they get older. I bought a slant six 70 challenger for $800 when I was right out of high school now that same car would cost at least $3500.
There is a 68 D100 custom that I am looking at getting right now. The guy wants $1300obo and a few years ago I wouldn't pay over $600 for one.
Doing auto appraisals it surprises the hell out of me when I get a call on a car and I think that it won't be worth that much and when I run the comps on it it is worth 5 times what I thought it would be.