~10K for an effective single-dose treatment is not that expensive. Admittedly this is a very crude calculation, but it's probably not too far off. Moreover, mAb production is a very mature technology, that can be scaled up by just growing more cells. I'm not saying we are close to getting this into the clinic, but it is a very interesting result.

Across 100M people that's a cool trillion dollars.

True, but even if we're only talking dollars, we'd have to compare that trillion to the economic amount lost by not inoculating.

There would be a lot of dead people but the economic impact of those deaths would be pretty insignificant because highest fatality rate is for people in their 80s.

I think you underestimate the psycho-social effects of having millions of deaths.