Alright, so at present, Hashflare is selling 10GH/s at $1.40 for their 1 year contract with a maintenance fee of 0.0035$ per 24h per 10GH/s.

Assume I buy 1 TH/s (100*10 GH/s) at $140. At present I get approximately 0.00018430 BTC per day at that rate.
This 1 TH/s keeps mining for a whole year, so I get  0.00018430*365=0.0672695 BTC by the time my 1 year contract ends which at the present rate is around $550. The maintenance would be $0.0035*100*365=$128 approximately.
So, I end up with $550-$128=$422 for an investment of $140.

Assuming that the price and difficulty remains constant, that's an ROI of 300%! Of course that's impossible. What did I miss in my calculations?