Depends on the size. I have seen small ones go for as cheap as $20, but they were about 1.5" long. I have seen them go for $300 also, 5.5" Baryancistrus Dematoides.
The Dematoides goes for more money, so if you buy one of the Subvirdis variety, it will cost less. Both of mine were $45, but in excellent condition, and nearly 4".
Is the dorsal fin's membrane attached to the adipose fin? If it is attached it is a Dematoides. If the tissue isn't attached like that, but they touch than it is a subvirdis.