I work at a record store and I have found a couple mono promos recently and they go for some big bucks. I just found the donny hathaway & Roberta flack mono promo and it has sold for almost $300 while the normal us pressing goes for like $5. They seem to be worth way more than any other white label promo i've found. Can someone explain to me why they go for so much money? Whats super special about them? Thanks.