Collector value, I would imagine. I paid $80 for Gnome Press' Conan the Conqueror from 1950. The story is essentially the same as the one in the Lancer/Ace editions from the 60's and 70's (of which I have three copies), the Donald Grant hardcover from 1989, the Millenium edition, and the Ballantine edition of "The Hour of the Dragon" (not to mention SciFi book club's "The Essential Conan"). I have more copies of that story than I can shake a stick at, yet I still paid $80 for the Gnome Press edition.
The reason they call us "fans" is because the word is short for "fanatic." Although I don't own the GURPs Conan stuff, I can see why someone would pay good money for it. My Robert E. Howard bookcase is pretty full, and many of the books repeat stories I have elsewhere. (I have three copies of "The Coming of Conan the Cimmerian" - A shelf copy, scifi bookclub's version, and a copy I actually use while doing research for Mongoose's Conan products.)