Why are Superbirds worth so much?

My brother had a Superbird in the early '80s. It was an awesome car, and as someone else said earlier, after about 100 mph it would srart to squat down and stick to the road. Now that's an awesome feeling.
He bought it in '83 for $8000, and sold it in '85 for $8000, he kept the 15 by 7 rallyes and the broadcast sheet, the new owner didn't want them believe it or not! The guy still has the car. 440 auto console lemon twist.
Hey, post #70 on this topic, good luck or what?