It sounds like a Lysiosquillina sp. If so that price is in the ballpark (if you find someone who actually knows what it is and wants it). As someone above mentioned this guy needs a really deep sandbed (8-12 inches) based on the size you mentioned, and deeper than that as it will grow larger if taken care of.
I don't really know why anyone would want one of these unless they're really, really into collecting or studying mantis shrimp. Many of the smaller species are interactive and don't need the specialized setup.