It's not even all that difficult to estimate, really. For example, every business does it, of necessity.
Markets are a useful tool that societies use to optimize certain kinds of capital allocation and goods production, where that makes sense. No more no less.
Most of the things societies value most highly don't fit into the market hole.
If trains were exclusively a mechanism for capturing or avoiding carbon, then the metric of $ / tons would be valid. Trains very obviously are not that.
Where is this true?
https://en.wikipedia.org/wiki/Value_of_life
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.
This isn't even true. I run a business, and I frequently make decisions based on what I believe to be the right thing to do rather than what the data shows as being the most profitable outcome. The very notion of a "loss leader" is driven on fuzzy data where execs simply hope the loss in revenue from sales is made up for in other benefits that can't be concretely valued.