$0.08 /kWh is good price. If you buy the 6x R9 390 and a mother board with 6 PCIE slots, you might ROI.
PSU probably will cost a lot!
And if GPUs is easy to sell on ebay - PSU not
PSU is not easy to sell on Ebay, it can only get 25% of the original price. For the good graphics cards, the depreciation is low.