First what is the guy asking?
second make sure it is the real deal. I have seen a few of these heads come up in the past few months. It is a prototype but what is it? is it a wax sculpt, hardcopy, is it handsculpted or was it cast? Is it a sculpt of the final design or an early sculpt. The auctions like Dale pointed out are a good way to see if it was up for sale recently or similar ones have surfaced.
Also try to get intouch with John Hamm (daddysnaggle)
he has been after a number of Jabba items as of late he may be able to give you insite into this piece if he has ran across them before. I don't know if he'll have the answers but he would be a good place to start. maybe even James Gallo something seems to lead me to beleive he had some jabba stuff in the past. I could be wrong though.