Please show me the math. If you are not accounting for increasing difficulty, then don't bother showing it.
I did the maths,
in this thread, even at the time, with no difficulty change and no electricity bill I couldn't see how you could get a ROI in 6 months, (blackarrow was mentioning 4 months at the time).
I don't think you can get anything less than 12 months, and at the current difficulties you might be lucky if not 18 months.
FFMG
I kind of misread your original post. I think we're in agreement that these are a terrible investment if you're looking for positive ROI.