The value of antiques has been going up and down in cycles for long before the muscle car had more than everyday used car value.
Anyone old enough to remember how the prices of anything deemed "old" jumped up during the Bicentennial Celebration years of the mid early and mid 1970's? Almost overnight there were antique shops popping up everywhere - mostly full of junk. And tag sales gained rapid popularity.
My ex bought a working Singer 4 drawer cabinet, treadle sewing machine- complete with oil can, book, and attachments- for $2.00 at an auction house in Hebron Connecticut in the early 70's. And, two guys there laughed at her and told her she was crazy for buying a sewing machine without a motor. Five years later, the same model machine brought $250.00 at the same auction house.
And, the clunkers for cash program was a drop in the bucket compared to what ten years of the depression leading up to the scrap drives and gas/tire rationing of WWII, number of big classic cars of the 1920's and 30's that were sent to scrap yards. Many of those big Packards, Pearce-Arrows, and Cadillacs were bought up by the Japanese in the 1930's and shot back at us in the 40's. Then we scrapped even more of them to shoot at the Japs.
Now, with PBS and all the cable TV antiques shows sensationalizing and making the search for antiques look so easy and lucrative, there's another upward tick in prices.