Even though your 1080 Ti does 700h/s, it costs $700. A pair of old 7970's costing <$200 does the same speed. They would use double the watts but with decent electrity rates it would still end up being more profitable in the long run.
My profit model includes:
1) sale of gear. I have customers waiting.
2) density I have limited space.
3) cost of power my winter rates adjusted for using the heat are 9 cents my summer rates adjusted for extra cooling are 20 cents.
I am pretty happy with the price as I can do 700h for 210-220 watts 1400h for 420-430 watts
So during winter rates which end June 1 the pair of cards cost me 31 dollars a month about 1 dollar a day
during the summer the pair of cards cost me 62 dollars maybe 65 dollars a month say 2.05 a day.
1400 h is about 9 usd a day in earnings.
so I make 8 bucks a day in the winter and 7 bucks a day in the summer
By the time it is Oct 1 and back to winter rates I should be at 840 + 400 = 1240 cash back on the 2 cards. I paid 1484 for them so on OCT 1 they will be at 264 net
I will be selling them and should be able to find buyers for them at 400-500 each so 900 - 264 = 636 profit on 1484 invested in under 6 months.