$25 WITH power is NOT a reasonable number, unless you are using total JUNK for power supplies or used parts - but even the BARE CARD figures on the 1080 ti keep it close to the other GTX high-end cards on cost efficiency unless YOU are getting charged insane amounts for the 1080 ti compared to the others.
SHOW ME a system that manages $25 per slot with NEW parts - I really would like to see that.
Then show me what kind of crazy rip-off pricing you're seeing on 1080 ti cards vs 1070/1070ti/1080.
Ain't my job to build your rigs for you but its easy enough - get one of them 8 slot integrated cpu/ram mining boards from a random chinese estore, add 2 midrange PSU's and a little ssd and you're done. 1080ti's can be had for ~700 and 1070's for ~450 over here. Comparing to a 1070 is silly though when there's piles of ~220 polaris cards clogging up every local auction site and the prices keep falling. Was pretty much the same price relationship when the card prices were still high, just around 50% higher.
2 650 watt power supplies from a QUALITY maker alone are going to be $200 give or take $20 - which already uses up your $25 per slot claim.
There is also the VERY POOR COOLING of those 8-slot motherboards to keep in mind, unless you're running water-cooled or hybrid MORE EXPEN$IVE cards you're going to have MAJOR issues with high heat on anything past a RX 460 or GTX 1060, or you're going to have to add a lot of high-power fans (and MORE power draw to run them, as well as the cost) to keep the GPUs even close to cool - adding to the "per slot" cost of the rig.
I don't see $220 Polaris cards around here, the lowest I've seen for decent mining GPUs in the last 9+ months was $200 for one seller with a couple used R9 290 cards (and a phone number they NEVER got around to answering, so I don't know if the posting was legitimate) while the Polaris cards have been listing for MORE than Newegg NEW pricing the past month ($339 lowest current offering on Newegg, a Sapphire Nitro+ model, while most of the Craigslist postings have been over $450 with a very few between $400 and $450 on ANYTHING Polaris).
I suspect that's due to this area having VERY CHEAP electric though - people are NOT shutting down rigs here - while your area has quite a bit higher electric pricing.
On ZEC, a 1080 ti will easily exceed 680 sol/s with good efficiency and can get to almost 800 on good-cooling cards if you push them, while 1070s struggle to beat 400 sols with good efficiency and you're VERY lucky to beat 450 sols/sec by much when pushed hard - the performance vs cost numbers USING YOU PRICE FIGURES work out pretty bloody close.
That's not true, 2x650w corsairs cost ~90. The cooling isn't that bad either, the hottest cards I have are the single fan nitro 570's and they didn't go over 70C last summer after I installed some ghetto ass cardboard vent shafts.
Also most 1070's can reach 500 sols, not to mention they can do other algorithms like daggerhashimoto or neoscrypt much better relative to cost than a 1080ti.
The power cost is on the average ~0.15/kWH here but anyone can get it down to 10 cents if they draw over 15MWH monthly or even a few cents lower if they are willing to move.