I think its a very complex market which is partly driven by speculators, especially when other forms of investment are so poor.
The true value, whatever that is, depends on the numbers of that car which are available, and the number of people with the ability to pay the asking price.
Borrowing money to buy a high value classic is risky unless you're at the top end - 250GTO for example.
What's odd is how cars such as the E Type, which is plentiful, have reached such ridiculous prices. Much the same can be said of the air cooled 911's. But I suppose within each group are the limited runs, and the rest get pulled up behind.
I wouldn't want a classic which I couldn't mess with, or take on a touring holiday, but that's just me.
Its a funny old world.
