[wayner]

Some of the reasons that I see cars as valuable

(from lowest to highest with lowest and most temporary at the top, and the ones in the middle, the order could be changed)

1) high school car. I always wanted one
2) Price of admission to the club. (buy a first generation McLaren and you get exclusive access to their track events around the world, and they'll take care of and bring your car there for you, or an old race car with pedigree lets you run lemans historics.
3)Collectible value - its a rembrandt (250gto obviously now is fitting into this and other categories, but a 356 speedster is also moving form high school car to rembrandt. I wasn't born when you could buy a new rembrandt, but Im told I need one in my collection)

limited production 1973 911 cars sort out like this from most desirable to least
1) A significant race winning RSR
2) any other RSR
3) a lightweight RS
4) all of the other 73 RS cars
5) All of the S cars
6) more recently the E car are seen as better than the T but they used to be lumped together
7) A "T" with S options
8) a run of the mill T
9 ) A 911 with a 356 motor

Obviously this simple list needs to be 3 dimensional to account for various factors such as condition and pedigree and fame etc. The question is what has the market determined make a car special, and in the Porsche collector world, low production number is certainly one of them.