This paper estimates that $250B would reduce biorisk by 1%. Taking Ord’s estimate of 3% biorisk this century and a population of ~8 billion, we get: $250B / (8B * .01 * .03) = $104,167/life saved via biorisk interventions.
This estimate and others in that section overestimate the person-affecting value of existential risk reduction, because in the course of the century the people presently alive will be gradually replaced by future people, and because present people will become progressively older relative to these future people. In other words, the years of life lost due to an existential catastrophe relevant from a person-affecting perspective—and hence the person-affecting value of reducing existential risk—will diminish non-negligibly over a century, both as the fraction of people who morally count shrinks and as the average life expectancy of people in this shrinking group shortens.
(Adjusting the estimates to account for this effect would strengthen your point that prioritizing existential risk reduction may require assigning moral value to future people.)
Interesting points. Yes, this further complicates the analysis.