Depends on where you go (private or commercial)... and prices fluctuate.. If you run into Age of Aquariums at the reef conventions, you can definitely score platinum clowns for as low as $150... here are some rough estimates.. if you're paying more than this, you're probably getting ripped off (unless they're proven breeders)..
Platinum ~ $250
Grade A - 100-200
Grade B - 50-100