Not really sure on the rarest BP, but I think there is only one Panda Piebald in existance, so far. I think it costs $18k.
For the most expensive I'm not too sure on, but I've seen Banana Clowns at $40k. To me, the Banana Clowns aren't that attractive. I think the Ultramels are a lot more attractive than the Banana Clowns.
There was a partially scaleless BP picked up at a reptile show in Louisiana for around $10. It was sitting in an unopened bag of import babies. The seller told someone they could dig through them all if they wanted, and even seen the snake before selling it. It's entire head is scaleless, and has some scaleless areas along the rest of it if I recall correctly.
**EDIT**
Nevermind, the one from the import bag only had a scaleless patch on its head. I do recall seeing one with a fully scaless head and partially scaleless body, though.