The calculation is the simplest there is.

108 USD total investment / 14 USD a month ≈ 7,7 months payback.

As I have tried to get through here, this is in large a result of +30% increase for bitcoin and a huge spike for Litecoin. Without those it would not be profitable. I do not see why increased difficulty would deminish the probability for success, as these assets has to be looked upon as commodities. It will be compensated by a higher price.