AMD's RX470, 480 and 570 and 580 are the most 'powerfull' GPU's to mine ethereum, that's why they are out of stock everywhere. The second alternative now is GTX 1060 and GTX 1070. The are not powerfull as AMD's to generate ethereum, that's why they price stood normal till the miners dried up AMD RXs stocks. The GTXs 1060/1070 are more power efficient, so in places where the energy bill matters more, people are using Nvidia cards to mine Ethereum. The GTX 1080 price's didn't change because it uses DDRX, which is not efficient for the ethereum mining algorith. If 1080 where efficient, it would be costing 1000 dollars by now. And about the lower cards, GTX 1050 and the RX 560 and bellow, they are not efficient enough for the ethereum they produce to cover the energy they use, that's why their price still untouched too.