The R290/R290X didn't moon in price until after launch when their true mining performance became known (same as Vega).The R290 was well expensive to start with due to mining, almost £400!!!
It was actually kind of funny, IIRC OCUK had a £400 firesale on the HD7990 to get rid of the stock, then *BOOM* the next month they were going for £700 on eBay due to mining, you could literally sell them and buy TWO R290 cards, which a month later were worth £400 each on eBay, crazy times xD