Unfortunately rarity doesn't always translate to value with cars. I doubt if a flat-top would ever be worth more than a more common Vista Cruiser. Could be if it was some sort of special order or very highly optioned. How is the paint and interior, run good? I have seen them for 2-4 grand in that condition. Check the NADA and gold book sites for values.
Just my two cents worth, take it for what its worth.
