kebko & Carl's comments are largely compatible: if nets cost $1/person and save 1life/$1000, then giving nets to all billion Africans could save a million lives.

There is a serious problem if there is overlap between the popular interventions and those that are best--popularity should drive the intervention to diminishing returns. At least, I think so, but I don't know the numbers; I'd guess a billion has been spent on malaria, but not on nets specifically.