Even factoring a 500% increase in difficulty by mid-march 2014, you are still looking at $160 a day with Bitcoin at their current price in the $390 range.
500% increase in 4 months looks quite low. It is around 50% per month.
The math irony. 1.5^4 ~= 5.0
Still I think the long term hashrate growth will be more than 50% but your statement actually confirms the OP claim.