Example 1: hashnest.com
Price per GH/s: 0.00135BTC
Maintenance fee: $0.0032536/GHS/Day
Current BTC->USD exchange rate: $400

Purchase 1TH/s for 1.35BTC.  As of right now, that expects to earn you 0.01686BTC per day.  At current exchange rates, that's $6.74 per day.  Subtract out maintenance fees of $3.25 per day.  Net earnings per day is $3.49.

Now let's extrapolate over time using 20%, 10%, 5% difficulty jumps:
20%: -1.192BTC.  Cost of fees outstrips earnings after 53 days.
10%: -1.004BTC.  Cost of fees outstrips earnings after 109 days.
5%: -0.6203BTC.  Cost of fees outstrips earnings after 208 days.

Even with 5% difficulty increases you have no prayer at current exchange rates. 

Let's do the same calculations assuming BTC miraculously recovers and hits $750 today:
20%: -0.3115BTC.  Cost of fees outstrips earnings after 104 days.
10%: 0.1492BTC.  Cost of fees outstrips earnings after 203 days.
5%: 1.071BTC.  Cost of fees outstrips earnings after 385 days.

If by some miracle BTC skyrockets today and holds at $750, you still need to bank on having 10% or less difficulty increases to actually turn a profit.

That a good enough set of calculations for you?