1. According to the FAO, 20-25% of CO2 emissions arise from deforestation.
2. According to the UN, between 1.8 and 2.5 billion people do not have access to electricity. They burn wood and animal dung for fuel. The wood they burn is deforestation.
3. Using wood and animal dung is both inefficient and extremely unhealthy. The UN Millenium Development Report in 2007 says that their use as fuel in homes causes more deaths than malaria.
4. One study in Maputo that is widely cited estimated wood consumption as household fuel at 1 cubic meter per person per year (about 800 pounds, or 362 kg). This provides between 7,600 and 9,600 Btu per pound in energy. To save a lot of research, if we estimated that 2 billion people burned 800 pounds of wood each year, yielding between 7,600 and 9,600 Btus (split the difference for average yield of 8,600), yielding per capita consumption of 6,880,000 Btus per year, times 2 billion gets 13.76 x 10 to the 13th power, or 13.76 followed by 13 zeros.
5. These people would only need to consume about 160 gallons of oil per year to get the same energy, as oil is much more efficient than wood. The oil burnt would also produce about 65% of the CO2 as that produced by burning wood.
6. In 2007, America consumed 86.24 quadrillion btus, 22.7 quadrillion of which were produced by burning coal or 86.24 x 10 to the 15th power, or 15 zeros. Rich people use a lot more power.
7. Half of America's CO2 emissions come from coal-fired electricity generating plants. As noted above, 20% of worldwide CO2 emissions come from deforestation. Let's call them roughly comparable.
8. I have seen one estimate of $150 billion for replacing American coal power plants with high efficiency natural gas, at the Energy Information Administration (but I lost the link...). This would reduce, but not eliminate, CO2 emissions by about 50%. It seemed certain from what I read that other alternatives would be costlier, especially if intended to reduce emissions by more than 50%.
9. In the past 25 years, an equivalent number of people (2 billion) were provided with access to electricity. But most of them were in cities, and I cannot find how much was spent on this.
10. The cost of providing solar power (much the most expensive possibility) to the remaining third of the population was estimated at $400 billion. However, solar power is not overly welcome by users, due to variability of output, and the cost does not include appliances to use the power.
11. A feasibility study by the DFID estimated the capital needs for energy infrastructure in Africa alone at $11 billion. Multiply by three to cover the rest of the developing world.
Back of the envelope calculations show that providing energy to the poorest third of the world's population could possibly cost one third to one half of the amount estimated to reduce America's CO2 emissions by one quarter. The additional benefits to longevity and the quality of life in the third world make it a compelling alternative to moving from coal fired plants to natural gas in the U.S.
Worth a bit more research, I'd say.