Some of the more expensive breeds are the ones with the leatherback or silkback traits.
Leatherback is a genetic disorder which makes the scales small and makes them have a 'smooth' appearance.
Silkbacks are the result of two leatherbacks mating to create essentially a super leatherback which we call silk backs because the scales they have arn't visible or they don't have any, they kinda look like a smooth salamander and a bearded dragon mixed together.
Here is a leatherback
And here is a silkback