I believe the $2M = 2500 lives saved number that julia is using comes from the givewell.org evaluation of village reach:

http://www.givewell.org/international/top-charities/villagereach

It's a pretty thorough review of the charity, and givewell understands that spending $100 on vaccinating N children does not mean N lives saved. They write:

"These assumptions yield an estimate of one additional child fully immunized for every ~$41 of VillageReach's expenses ... this would imply that VillageReach is averting a child death for every ~$545 it spends"

Julia's calculation appears to be using $800 per life saved, which is even more conservative.