It is not because they are powerful. It is because a lot of them together are powerful for mining. The miners started moving to the 470s after the 480s started going up in price. I do not know much about mining but that is why the cards have gone up in price. The difference in performance between the 470 and 480 is not that much so the miners use them to almost the same extent. I just wish I knew how to sell my 470, I would probably look for a used card like the 970 or save for a 1080 if I can sell it.
I'm looking into the same thing now, I would love to get a 980ti or 1070 depending on which one is cheaper. I'm about to look into Ebay and see about that. (Btw thanks for the info)
https://www.newegg.com/Product/Product.aspx?Item=9SIA24G4VH6414 Look at this 470
That is a stupidly high price. For that price you could get a used 1080ti. Or be a few dollars away from one. The 980ti would be the cheaper option and the better one because of it. The 970 is 10% faster than the 470 and it goes for about $170 used. If I find a away to sell my 470 I will probably get one and wait pocket the rest of the money wile i wait for Volta.