It is a rather sad and naive view of the value of work if you think that it is decided by market value. My work as a developer has a high market value because what I do is generating revenue for my employer. The work of the teacher who shapes the future of our children or the work of nurse that take care of our dying parents don't generate any revenue, and thus has a lower market value, but I, and I think we as a society should, value it much more.
Good teachers and good nannies aren't easier to replace than good carpenters or construction workers. But since many men who are in charge of setting salaries don't understand that they aren't as well paid.
> Have salaries gone down now that women make up a significant percent of the number of doctors?
Yes. I don't have the time to dig up the sources right now, and I don't remember seeing it specifically about doctors, but in many lines of work the relative salaries have gone down when traditionally male professions have been taken over more and more by women.