Are you talking street cars or race cars? Barett-Jackson has destroyed the classic/muscle car market. They have provided a false since of value based on rich men who have "mine is bigger than yours" attitudes when it comes to check books. They look hard and really qualify the buyer. They take the biggest check books that have the gotta have it attitude and put them in 1 room with complimentary alcohol then put it on TV. Next thing you know, the cars that the average person passed on are cars that the average person can barely afford.
As for race cars, there are no standards. It based on what you have in it, what you think it is worth and what the next guy is willing to pay for it.