My #1 most wanted knife is an Emerson NASA. This is a true "grail" in the sense that I will probably always want it, but never have it. Not very many were made (I don't know the exact number, but my guess is between a handful to a few dozen).
There's nothing special about this knife in any practical sense. In fact, with the cutout in the forward tanto portion, it's probably less than practical for general purposes. But...it was commissioned by NASA...they've been in space...so it's got both the cool factor and the rarity factor going for it. And I'm an Emerson fan anyway. So yeah, this is my grail.
I can't find any completed listings on eBay and I can only find one story (by a quick google search anyway) of a civilian possessing one. My guess is that if one ever comes up for sale, it will go for thousands of dollars.