currently difficulty is at 37. and is estimated to jump to 50. which is %40 increase so, if this trend continues, we are talking %120 increase per month or %240 per 2 month
that is why I decided not to buy knc Jupiter at the moment, even if it is delivered in October as promised I still would not make ROI.
here is a better calculator
http://mining.thegenesisblock.com/even if you assume difficulty will increase %35 per 10 days and round it off to %100 per month the outlook is bleak