The problem is jobs aren't homogenous, so comparing salaries is meaningless, not only for applicants but also for the company itself. Imagine you find 10 people whose resumes look similar on paper, but after you interview all ten you realise there are some you'd hire in an instant, others you think are just okay, and some you think are awful. It's so obvious why their salaries would differ, and I find it very challenging to make any good argument otherwise.

So, how can market participants ascertain supply and demand (and hence, price)? The answer is they can't, but they're no worse than companies, academics, government or anyone else - without assessing the individual, I don't think it's possible for anyone to know.