I could use $4000 and get about 6 BTC right now. If I use $4000 to purchase a mining contract at 370gh/s I would earn about about 14 BTC over one year if the difficulty remained the same as today. We know the difficulty will increase greatly, but will it increase greatly enough so that under this contract I would not even earn the 6 BTC that I could buy right now?
Just remember that in january this year the hashrate doubled, just like that. There is no reason why the hashate would not increase much much more this year unless something dramatic happens. It's an exponential growth curve so whatever we think is a lot today will be almost meaningless in the near future.
Thinking in terms of difficulty staying the same is just wrong on so many levels. You should leave that out of your reasoning completely because it gives you a completely wrong prognosis. Don't use it as a basis because it's absolutely not like the real situation.
With the contract you will likely lose out. Just as a base case, if the difficulty rises only at a rate of 20.00% per month (very slow) and if you do not have rrp then you will make about 7 BTC in a year.
But that is just about the best situation imaginable. It can only get worse and the difficulty will probably rise much faster and you will get much less than 7 BTC...
Just think about it, in 2013 difficulty increased roughly by 100000.00% (!!) .
. And you need less than 20% to make more than 7BTC.. It's not worth it because the risk is way too high.