You can see the spreadsheet model GiveWell used to come up with their "dollars per life saved" number here. See the "Bednets" tab for the AMF calculation. With regard to how the foundation spends its money, the only input considered is "dollars donated per anti-malarial bed net distributed" (about five bucks, depending on country), and work proceeds from there to establish the value of one bed net distribution.

https://docs.google.com/spreadsheets/d/1zLmPuddUmKsy3v55AfG_...