I'm pretty skeptical of the $50 number. GiveWell thinks it's closer to $1000 (still insanely cheap, but there might be even better opportunities!)
https://www.givewell.org/international/technical/programs/ca...
https://www.givewell.org/international/technical/programs/ca...
Givewell agrees with the $50 figure.
"[W]e suppose that a program costs $5 per person screened, that 20% of those screened receive cataract surgery at a cost of $15 for transportation and $25 for the direct cost of surgery, that 10% of those who receive surgery are blind or severely visually impaired, and that surgery is 100% effective. This program spends $650 per blindness or severe visual impairment reversed. We are highly uncertain about how close these estimated values are to the true costs and uptake of programs."