[jjflash]

Hagerty's (the classic car insurance company) has a "Cars That Matter" website and subscription service that attempts to track prices on a wide range of "collectible" cars including Porsches. Here's the website:

Hagerty's Cars That Matter: Reliable prices for collector cars

I subscribed for awhile, but don't really have a use for it now. They track models by condition, with 4 levels of condition. I don't think they try to quantify "collectibility."

They track LOTS and LOTS of auctions, so in that respect they have actual prices on cars. Getting that data--he actual prices at which cars change hands--from private parties would seem to me to be just about impossible ;-).