So at current difficulty you would get 256 dollars per month minus 2,925 per day is 168,25 dollars per month of income.
That's an ROI of about 10 months if the difficulty doesn't rise.
So since the difficulty does rise....what is the average rise in difficulty? I mean, how can you calculate a ROI date properly, is there an average percentage the difficulty goes up each year for example, that can be use to guestimate ROI?