It doesn't particularly matter what the specific number is, what's really important is that we have consistent way of looking at all the different ways we could save lives by spending money and evaluate them rationally, rather than spending $1e9 to save a life here but refusing to spend $1e5 there. People's intuitions about these things are really horrible, and if someone is willing to pay an amount to save 1 life they usually aren't willing to pay anything like 10 times as much to save 10 lives.