The UN's Intergovernmental Panel on Climate Change concluded today [2] that it would cost .12% of the world's domestic product to substantially reduce our collective greenhouse gas emissions.
GDP of the world economy: US$60 trillion
.12% of $60 trillion: $70 billion
Total population of the earth: 6.5 billion
Cost per person to significantly reduce heat-trapping gas worldwide: $10 a year
Cost of saving the planet from droughts, famine, mass flooding, species extinction and rising sea levels: priceless.
Note: I've revised the calculations here. From $110 to $10 per person.
Here's the math: $60 trillion/.0012/6.5 billion = 10 (rounded figures)