Your definition may be useful for cold hard utilitarian calculus, of the sort that hospital directors need to do if they've run out of fundraising opportunities. However, "effective altruism" – which I suspect you're alluding to here – isn't actually an efficient way to save lives, the way it's usually practised (ignoring second-order effects, and everything that doesn't fit on a spreadsheet).
"A review across 23 LMICs found that low-complexity surgeries (e.g., appendectomy, hernia repair) cost only about ~$17 per DALY, whereas even complex procedures were often cost-effective" (Most surgeries on Watsi are low-complexity)
"Reports from the WHO and Lancet Commission consistently emphasize that investing in surgical capacity has high value, in many cases, more than essential drugs or vaccines on a per-DALY basis"
Both quotes from: https://link.springer.com/article/10.1186/s12893-025-03204-0
That said, once again, Watsi is great. I really appreciate all the hard work they've put into making this happen—this is orders of magnitude more impressive and impactful than most projects I've ever seen!
I honestly have no idea about healthcare costs in countries like that, but assuming the main cost is a surgeon, the reported median salary for surgeons in Sweden is between 110-150k usd yearly. Assuming 200 working days a year and available for something like 4 surgeries a day on average that goes to about 137 usd per surgery. Adjust for Malawi being a far cheaper country but addition of more staff and a cost of 400$ for a relatively minor surgery doesn't seem too far fetched if there isn't middlemen taking a cut (actually googling it seems like middlemen can take a fairly large cut even with those costs).