Question: why are gtx 1070 so expensive?
Answer: The 1070 would never cost more than any 1080 , but recently due to the etherium mining craze (it's like Bitcoin) it seems many GPUs like AMD's Rx 480/580 ,570/580 ,1060 and the 1070 have sudden increase in demand. This is because these graphic cards are optimum for etherium mining.