Depends on what the outcome is. If it makes the site 50% more performant on 25% less hardware, pretty easy to swag it. Same if the outcome makes developers on the team able to ship new functionality 20% faster with 33% fewer bugs.

This seems awfully contrived.

Issue 1: It's very difficult to tell if your contribution got 50% improvement in performance because there were 10 other devs pushing in features and bug fixes. This is the attribution problem

Issue 2: This happens over time. It's very unlikely that your 50% improvement happens every year or month. Because, think for your self, this is compounding with large rates. It grows quickly. 1.5x improvement in 6 cycles (months or years) is 10x. This essentially is the time problem

Issue 3: even if you deliver the results you did, in a large company there's a large bureaucracy and no one person has the ability to increase your salary by that much. This is the control problem.

The problem with this argument is that programmers don’t work alone in a vacuum. How do you account for the support staff? The recruiter that hired you? The cleaning lady? The DevOps people? And so on.

It’s avtually fairly non-trivial to be able to say with even a modicum of certainty how much value a given developer brings to their company.

This is precisely my point! Thank you for getting it and explaining it.

I currently write software used by millions of people. Partly because I’m a backend engineer, I have no real idea how much more the company is making due to my direct efforts. Since they keep paying me, I’m assuming it’s a decent multiple of my carrying cost, but I have no way to measure it.