No. Sables are not rare. They are usually working line. No, they are no more expensive than any other GSD.
Yes. There are breeders out there that claim they are rare and sell for more. When Jax was a baby a guy stopped me and was telling me all about his "rare" GSD.
There are no differences health wise or temperament wise between the black/tan or sable.
Yes, there are differences between show line, working line and differences between litters depending on the genetics.