It seems incredibly bizarre to assign a monetary value to the elimination of all human life given the concept of monetary value would be wiped out along with the people.
So you have to put the monetary value somewhere (although you're completely within your right to question this specific amount).
My immediate reaction though is to doubt the mapping of dollar to value - e.g., the 10 million dollar valuation of the human life, but also the valuation then of all the things that year-dollar-cost could be spent on. Many of those things probably don't map very well between true value, and dollar cost (my go-to example of this is teachers fulfilling one of the most critical roles to ensure a functioning society, yet the dollar cost paid for their labor being typically far lower than most other jobs).
And indeed, accounting for externalities (unmeasured or unmeasurable) is a tough economic proposition. If it weren't hard to account for every single variable, creating a planned economy would be easier (ish).
FWIW, there's a whole sub-field just dedicated to determining the value of life for various purposes (a starting link: https://en.wikipedia.org/wiki/Value_of_life). You may disagree with any specific assessment, but then you have to argue how that value should be calculated differently.