http://I got 100% free electricity.. do u think, if I buy antminer it will be profitable to me.
what will be the lowest and highest btc/usd I can make in 1 month?
With 100% free electricity, chances are pretty good that some day you'll break even, or even make a profit.
That being said: it might take you a really long time, depening on the costs (you'll have to buy an S7, make sure it has a suitable place to run, buy extra's like PSU, cables, shelving, ventilators). You'll need to make sure the wiring is ok, the temperature, humidity and noise are all under controll,...
If the antminer breaks before you break even, you might even end up losing coins.
You can run simulations over here:
http://www.coinwarz.com/calculators/bitcoin-mining-calculatorTake into account the diff rise that might happen because the S9 hit the market, and the fact that the blocks will halve really soon
I ran a really quick simulation with a block reward of 12.5, and at current diff and bitcoin price, the estimated "profit" would be around $3.5/day (this also depends on the S7 batch you're simulating, and the PSU).
Take your total costs, divide it by 3.5 and you'll know how many days it MIGHT take you to break even. If the diff rises or the price drops it might take a lot longer. If the antminer breaks before the days to break even, you'll lose part of your investment.