Why are Superbirds worth so much?

I heard that the Plymouth dealer here in my area couldn't sell them. So they took the wing and front end off and converted them into '70 Roadrunners.

Like this:
http://www.carsinbarns.com/Superbirds & Daytonas/pg24.html

Read post #8:
http://www.forabodiesonly.com/mopar/showthread.php?t=56085&highlight=indiana