The best paying per post signature campaign is DaDice right now if I am not mistaken. So you can theoretically you will earn max 0.13 by posting per week so 52x0.13=0.672BTC per year.
Let me be the one to correct your math:
52 x BTC0.13 = BTC6.76 per year.
But doing calculations like this aren't very accurate long term. In a year, the price can change a lot so im not sure the campaign owners will be nice enough to not lower the BTC payments if price goes too high again.. they should keep it because as of right now it doesn't pay much but yeah.. it is what it is, a small bonus.