... a 1GH rig would pull in $8/day or $240/month.
You will suffer a big loss if you calculate your mining result by multiplying your first day by 360 to get the year.
The increase of difficulty means that you have to multiply the first day by 30 (instead of 360) to get the full first year and that every following year will be virtually zero.
1 GHs is 20,300 shares/day on the average
Current difficulty is 1,564,057
resulting in 0,65 BTC the first day (=50*20,300/1,564,057) for 1 GHs
Difficulty last year
In the past year difficulty period was about 10 days with about 40% increase after each period.
Supposed the same will happen in future
1 GHs will produce 6,50 BTC (10 x 0,65) in the next ten days
Then the next difficulty increases will limit your BitCoin income
to the double of your first ten days.
Total income of 1 GHs (running forever) is 19,50 BTC (= 3 x 6,5)
if difficulty increase goes on with 40% steps.
Last increase was lower. So when you are lucky your 1 GHs running forever will produce a total of 39,00 BTC.
To pay back hardware cost of about $ 1,200 for 1GHs the BitCoin quote has to increase to $30,76 plus energy cost.
I'm confident that the BitCoin will increase even more but before buying a new rig you should be aware that this rig probably won't be profitable as long as the BitCoin stays at $15,00.