Studies / reports on increased wellbeing for extreme poor

I think I remember reading somewhere about how it costs much less to double the wellbeing of people in extreme poverty than it is to double the wellbeing with higher incomes. I don't remember where I read that though. Can anybody point me articles or reports on this topic? I'd be especially interested to see good studies that put some numbers to this and/or reports (from GiveWell maybe?) about how specific charities improve wellbeing.