Numbers vary around the world, but FEMA in the US decided on $7.5M in 2020.
Financial investments are economic decisions. Whether or not you like the idea of assigning financial value to fuzzy concepts like happiness or quality adjusted life years, you still have to do it. The simple act of choosing to spend $N or not choosing to spend $N puts an implicit price on the result.
By putting approximate numbers in the spreadsheet - no matter how crude - we can at least end up consistent and fair. Otherwise we could end up spending vast sums to make a few people a little happier instead of smaller sums to make lots of people a lot happier.
No, but we can look at the amount of misery, that is prevented by not using cars. Traffic jams, pollution, accidents, waste of resources,...
The difference is so huge, we don't need to put a number on it. Public transport might be unpleasant for the individual, but is clearly very much better for everyone than using cars.
