You say you currently make $540 per month, or roughly $18 per day. Over the next 11 days you will make $180, and then you will make 30% less, or $126, over the following 11 days, and so on. Let's look at what you will make (rounded to the nearest dollar):
Days | Income | Total Income |
11 | $180 | $180 |
22 | $126 | $306 |
33 | $ 88 | $394 |
44 | $ 62 | $456 |
55 | $ 43 | $499 |
66 | $ 30 | $529 |
77 | $ 21 | $551 |
88 | $ 15 | $565 |
99 | $ 10 | $576 |
110 | $ 7 | $583 |
121 | $ 5 | $588 |
132 | $ 4 | $592 |
143 | $ 2 | $594 |
154 | $ 2 | $596 |
165 | $ 1 | $597 |
176 | $ 1 | $598 |
187 | $ 1 | $599 |
Keep in mind that these numbers are probably not accurate. The difficulty has been going up at this rate for 8 months now, but it can't keep going like this forever. Eventually, it will have to slow down and it is not clear when that will happen.
My actual prediction is that the difficulty will continue rising at this pace for another 3 months before its starts to flatten out. I predict that it will go up an average of 20% every 12 days in the end, and each GH/s mining now will earn a total of less than 0.14 BTC. It is not quite as dramatic as the table above, but your 60 GH/s will earn only 8.4 BTC, which is less than what you paid.