I think its because I have a bit of a thing for botanical drawings that these really appeal, I particularly love the last wallpaper here, which looks like pages from a botanical book. Will certainly save on ripping pages from that book you found at the flea market and pasting them to your wall individually!
The wallpapers are priced from £65 per roll, which I think isn't too bad. I always try to use the logic of cost per use... and you will look at wallpaper in your home every day. I don't regret the money I spent on the wallpaper for our hallway as it gives me pleasure every single day! :)