For the sake of argument, I'll say it's way too high: Consider that a very plain, but large, squamosa can be had for as little as $60. That was mine, an 8-incher, and it's still going after three years.
Add a really great color variation, and maybe $200 or so to start? Then consider that at 10 inches, your acquaintance is perhaps trying to make room in his tank. Clams that large take up space, and suck up calcium. Why does he want to sell? Sight unseen, I wouldn't offer more than $250.