I haven't bought or sold one, but from what I remember, Shoehornfish are basically just a rare collectors item that doesn't have any uses. So the price, really, is whatever a collector will spend on it. If someone really wants to finish off their gallery, for instance, perhaps they'll spend 10 million, but someone who isn't as invested in their collection wouldn't.
It's really hard to give acctuate prices for items that like. If you're trying to sell yours now, I would put something put a price that is a little higher than you actually want to sell it for, mention that it's the lowest price, and say you're willing to haggle. Good luck!