I have limited free power - enough to run a few S7.  I bought these in December, and am no where near break even so far.  In your calculations, better factor in power supplies, shipping, down time (+++++), halving, etc.  Also factor in your time - the first time you work on this, it will take you a few hours to get everything setup and running smoothly.  You'll need to spend a few hours a week watching these machines, and verifying their output.

I'm sure I'm doing better than most, but the hardware producers (Bitmain) price their gear such that you will barely make a profit after running the gear for a year.  After that year, the machines are essentially out of date, and worth very little.