To be clear, what they bought is not a copy of the novel Dune in any form. It is the pitch book that was made for the film adaptation of Dune by Jodorowsky. So it includes a bunch of (reproduced) artwork by Mobius, Foss, and Giger, storyboards, script and story notes, stuff like that. It does not include the full text of the novel. 20 copies of it exist and one sold in 2019 for around 35k
You can see a sample of it here, though I understand it is possible to find scans of the full book if you dig around: Jodorowsky Dune Bible Book Sample : Alexandro Jodorowsky : Free Download, Borrow, and Streaming : Internet Archive (download the torrent or the zip file to see 46 pages of it, which are apparently worth a few hundred thousand dollars!/s)
