Don't forget that the difficulty [1] will increase over time. By using the calculator [2] with only a 5% increase, zero electricity costs etc. we don't see any return at all.
I remember a year or two ago I was calculating all my BTC mining efforts on its value being under $30. Some efforts were break-even at sub-$30 exchange rates, but now in retrospect were well worth it.
But the big term is may, as it also may decrease.
In addition, that second link helps us work out what equivalent hardware at home we could use to accomplish a similar (non)return. Purchasing an AMD HD7970 graphics card, with a hash rate of 0.7MH/s, and mining with that could arguably be cheaper in the long run
[1] http://www.forbes.com/sites/kashmirhill/2014/01/13/a-100-wor...