@bokel1
How did you measure the card alone at the wall?
Anyway, do a 6 card rig Rx 580 vs 6 card rig Navi and see where you are. And again, you have to consider also 2 x Rx 580 = 1 Navi regarding price.
Idling the rig is using 50-60W. Actually, idling RX 5700 is using less than 10W while rx580 is using 20-30W. As for the card purchase price, you are right. But I am rather concerned for the running cost...