Yes, it's much cheaper to save lives in the third world than in the first world, but that obviously doesn't mean that we should stop investing resources into education in the first world.
If you want to figure out the value of an education in the first world, you would have to take the cost of the education and compare it to the amount of added value that that person will produce over the course of his lifetime because of that education.
Obviously something like mosquito nets to a third world country is an area where not as many resources are being invested as should be invested, but that doesn't mean that you drop all other investments in order to only do that.
I'm not sure if your objection is that our economy should put nonzero resources into first-world education. But if it is, note that the issue is where the next marginal $5000 should go — whether, given the current allocation of resources, marginal dollars do more good in first-world education than in third-world health. If our hypothetical teacher/trader's donation is somehow matched by our entire economy, or even by all philanthropists, the AMF's room for more funding would be instantly exhausted.
...If you want to figure out the value of an education in th
A very interesting article on "earning to give", featuring LessWrong members Jeff Kaufman, Julia Wise, Holden Karnofsky, William MacAskill and Toby Ord. Some excerpts: