With the snakes I've shopped around for, 66%'s are usually a bit more than normals, but not as much as 100% hets. If I were to guess, I would go with probably less than 25% more than a normal.
Depends alot on the snake and the mutation in question. For example - a 66% het anery hondo would probably be priced the same as a normal hondo. The anery gene on it's own isn't widely sought unless it is being combined with one of the other mutations(amel, hypo) -- too bad cause I like Anerys...
A 66% het Pied Ball on the other hand would
not be priced the same as a normal ball...