Paolo,
I agree their are many replicas out there that look better than some originals, and they probably drive better too. And not having to worry about someone denting a fender adds alot of pleasure to being able to drive the car and enjoy it more.
Being able to bring in the big bucks has to do with the history of the car, and any special features the car has to offer. Shelby American assembled the Cobras on the assembly line with continual changes, thanks to racing evolution and refinement of the car. These changes sometimes added to the value of the car. Then if you add any special history of ownership, not to mention racing history, the value just keeps going up.
If you have the time, take a peek at some pictures of my Cobra here:
http://www.cobra-ig.de/anzeigenmarkt/fahrzeuge/forsale1964accobracsx2075.php