I would have to input and say probably popularity affects the price and also obscurity. Items like Deities and Demigods with Cthulhu are well known even to small time collectors. When you go up to them and tell them "hey have you heard of something called outdoor geomorphs" they have no idea. The Deities and Demigods is very well known throughout the hobby and people are mystified by how it was banned and will go on ebay and try to buy it. Everyones heard of it. Popularity also affects the price. For some reason opeople like the H modules so they are willing to pay a price to obtain it. Same goes for From the Ashes and Kara Tur. Neither are extremely rare but their are enough people out there wanting it that prices go up pretty high when you find one on ebay.