You're probably aware of this but it might be useful if not:
http://mining.thegenesisblock.com/The calculator is nice to play around with and the chart at the bottom of the page give some pretty good info on $/GH.
Isn't a 119% difficulty increase per month a little unrealistic?
With knc second batch shipping , Hashfast delivering my the end of the month , and many more in January February , no it's not unrealistic at all.