I know nothing about Yemeni paper currency but I wonder if like any collectible it might not be a case of supply and demand. This guy says he has 200 of these bills. Maybe other dealers have them as well and a huge stash of them was found? I don't know how many collectors of Yemeni currency are out there but I can't imagine that millions of people collect it so it might not take a huge find to cause prices to plummet.