I noted this in my post, but it's so long it's understandable if one missed it.

Yes, I'm aware that if we just pumped funds into malarial nets it would quickly lose low-hanging fruit to target (in fact, by being givewell's top charity it already passed the marginal point into 2nd place, and IIRC is now hovering around $2000 per life saved but I'm not certain on that so I'll stick with Yvain's numbers)

I'm not sure if malarial nets were never at 500/life efficiency, or if they were at 500/life at the start of their operation, then the charity got so much funding that all the low hanging fruit was picked and the price increased to 2000/life. My source was based on 'things I sorta half-remember from a newer Givewell interview' whereas Yvain had a concrete number written down, so I used that.