that barely adds up to 200THash, which is about 10 times the current network rate, and if difficulty goes up by a factor of ten using the same formula above. your ROI is 156 days.
honestly, it isn't that bad.
this would be true only in the case that the difficulty will remain unchanged for 5 months, and you know that it will not
real ROI will be, at best, somewhere at 8 months.