Never seen one, hard to say, if there's no sales results to be found and none available for sale elsewhere then whatever you feel happy to pay. If its too much for you, then it's too much.
If I was going by the seat of my pants as a seller, and just guessing, depending on artwork, I'd say anywhere between $100 and $300, but that's a complete guess without knowing rarity, artwork, when it was released there, etc etcetera.
Make him an offer of what it's worth to you and you can't lose.