[michaelkeenan]

Ok, I'm not particularly attached to the idea of AIDS prevention being the greatest charity of all. As mentioned, I just checked Wikipedia to find out what the Copenhagen Consensus came up with because I'm sure they put more thought into it than the few minutes I'd allocated for it.

That said, I'm aware that givewell.org rates charities for their effectiveness. Their top-rated charity is Population Services International, which works on - among other things - preventing AIDS and malaria. "We estimate that it costs PSI $650-$1000 to prevent a case of HIV/AIDS and $500-$2500 to prevent a death from malaria; across the organization, we estimate that it costs PSI about $650-$1000 to save a life." http://www.givewell.net/PSI

If GiveWell's figures are accurate, it seems that $650-$1000 is what it should take to save a life. $27 billion to prevent 30 million infections sounds about right. If you have a cheaper way to save lives, I'd love to hear about it.