Well, from looking online at prices that they generally sell for I would think somewhere around the $250-300 range is quite fair.
It all comes down to many different factors though like size, quality, original cost, etc. etc.
I guess it always comes back to what the interested buyer is prepared to pay bearing in mind that most people aren't prepared to pay the big money for a fish that you rarely get to see and should not be kept in a community tank in my opinion.
I know that in the near future I will be definitely buying some wether it be from hff or a private breeder, I have always wanted some and am prepared to spend the money when I have it.