The government thinks $10M per life is about the right number on the margin given US's economic situation and the slack they have in the annual budget. Obviously if there's a nuclear war tomorrow, or something else that drastically changes either the money supply or the number of people who need saving, the exchange rate would be different.

Realistically I think the value should be a lot lower, considering there are many interventions you could do that saves more than 1 life per $10M, even if we confine calculations to the US. (World-wide the marginal cost of a life is probably less than $10k). Of course you have to also take into consideration what is politically and organizationally feasible, so perhaps $10M is the government's marginal cost of a life considering its set of permissible policies.

...Or maybe somebody just pulled a number out of a hat to justify an expensive piece of legislation in the 80's, and nobody's bothered to update the number since.

This is the thing - it’s likely the exact value for a human life varies dramatically depending on the context.

That’s because they’re not usually attempting to assign a value to human lives in any abstract sense.