I am in astronomy and it is common practice to post papers on arxiv.org. Because of the need for journal subscriptions and the time it takes for papers to be published, this has become the primary way we keep up with the field. I would say this has revolutionized math and the physical sciences.

Perhaps one of the reasons for the difference between fields is that, at least in astronomy, our journals are supported by the professional societies, like the American Astronomical Society, and not run explicitly for profit.

I should also add that in astronomy, the authors pay a significant amount of to the journal when a paper is published. It is usually in the range of about $100 per page.