We all have artists we don't get the love for. (I've spent 30 years wondering what the Jim Lee fuss is about but at this point his market is his market. Others might not appreciate McFarlane, or Kirby, or fill-in-the-blank, but as non-participants in the market for their work our voices effectively don't matter).
Maybe you're not a fan of Grell, but he certainly has his fans, and the Longbow Hunters, when it came out, was a super successful and well received project that he will always be remembered for by readers at the time.
Its a cover, its Grell, its material he's known for (although not his best known material). Sometimes that's all you need. I agree that its not the strongest image he ever drew (although the girl is a nice touch), and I agree that it was a strong result. But Grell has a body of work beyond this one cover and that will impact pricing in the same way it does for every artist.