If you're buying all 12 snakes at one time, I would offer $50 each * 12 = $600. Since they are not of breeding size or age, I wouldn't pay any more. If you can get them for $40 each, I'd say you're doing well. If they were breeders, I'd say they'd be worth no more than $80 each * 12 = $960.

Good Luck!
Jim Smith