Yeah...I'm not sure I would call it "material" :)
Ok, it's the most expensive "thing" to produce but it's hard do compare with the other items in the list.
These rankings focused on costs regardless of utility and production/harvest method usually mix things as different as snake venom, cocaine and caviar.
I guess many people would answer diamond is the most expensive material of all, so it's still interesting to know this kind of things.