You're absolutly right and I just wanted to point out the importance of the system costs. We could also bring power consumption and manpower to the TCO-level, but this far we can say just comparing pure GPU prices does not paint the picture.
And by the way I made the mistake and used a $500 price tag for the GTX 1080 (like you in your first calculation?). The all recommended F.E.-edition comes for 50 bucks more, which changes the calculation. Same for the GTX 1070 ($400 instead of $350).
To point out the calculation:
System: $600
GTX 1070FE: $400
GTX 1080FE: $550
GTX 1080 Ti: $700
System with 2 GPUs each:
GTX 1070FE: 26.43 MH/$
GTX 1080FE: 29.41 MH/$
GTX 1080 Ti: 31.0 MH/$
So I guess the Ti will be the best choice in most cases, except when your budget is tight or you decide to choose really cheap hardware for the system. But you can always do the maths and make a choice on your own.
And by the way I made the mistake and used a $500 price tag for the GTX 1080 (like you in your first calculation?). The all recommended F.E.-edition comes for 50 bucks more, which changes the calculation. Same for the GTX 1070 ($400 instead of $350).
To point out the calculation:
System: $600
GTX 1070FE: $400
GTX 1080FE: $550
GTX 1080 Ti: $700
System with 2 GPUs each:
GTX 1070FE: 26.43 MH/$
GTX 1080FE: 29.41 MH/$
GTX 1080 Ti: 31.0 MH/$
So I guess the Ti will be the best choice in most cases, except when your budget is tight or you decide to choose really cheap hardware for the system. But you can always do the maths and make a choice on your own.