One of the cheapest ways to curb emissions in coming decades would be to provide access to birth control for tens of millions of women around the world who say they desire it.
U.N. data suggest that meeting unmet need for family planning would reduce unintended births by 72 per cent, reducing projected world population in 2050 by half a billion to 8.64 billion.
The 34 gigatons of CO2 saved in this way would cost $220 billion. However, the same CO2 saving would cost over $1trillion if low-carbon technologies were used.
Dot Earth has more, and here's the link to the numbers (pdf).