Does this thread take into account only commercially released albums? If you're lucky enough you can find "Promo/Not for sale" releases on auction sites and some store, too. I've yet to see an album rarer than
Baldur's Gate Dark Alliance - The Soundtrack (by the way, you forgot to add "The Soundtrack" in the album's title, look closely at the image)? Spotted it two times at eBay, with the bids going as far as 973$. Who knows where are the other 47...