Like you wouldn't imagine, I expect you'd be looking at around £4000 for a bolt on 4 string Thumb Shortie.
Personally, I'd speak to one of our extremely talented more local builders to see what they could do for you. The recipe for the Thumb sound is no secret any one can buy Bubinga, Wenge and MEC electronics...
Personally, I'd speak to Alan at ACG, he won't build you a short scale Thumb, but he'll build you something way better
Eude