Well... Yes and no 
You're correct: you should take the total build cost into consideration. There's a reason we do not offer the GTX 1070 in the Brutalis in a clustered configuration, it throws the Perf/$ and Perf/Watt ratios for the cluster way off.
On the other hand, it's not that simple unless we have a standard baseline build to compare against. Like, the Brutalis!
So in this case, the GTX 1080 Ti would be the best bang for the buck, not the GTX 1080
You're correct: you should take the total build cost into consideration. There's a reason we do not offer the GTX 1070 in the Brutalis in a clustered configuration, it throws the Perf/$ and Perf/Watt ratios for the cluster way off.
On the other hand, it's not that simple unless we have a standard baseline build to compare against. Like, the Brutalis!
Code:
N4X48-GTX1070-2620-128-2X500 20389.00 USD 147.1 GH/s 7.21/MH/$
N4X48-GTX1080-2620-128-2X500 21589.00 USD 200.3 GH/s 9.28/MH/$
N4X48-GTX1080TI-2620-128-2X500 22789.00 USD 243.7 GH/s 10.69/MH/$So in this case, the GTX 1080 Ti would be the best bang for the buck, not the GTX 1080