Okay, since I have enough of this no ROI crap, here is a little hint:
even at 700.000.000 difficulty you have an ROI. Granted, if you buy an asic at that point it takes 1 year and a half, but ROI is still possible.
If the cost of electricity(*) exceeds the value of the mined BTCs you´ll never have ROI.
(*) There is no such thing as free lunch/electricity