Walt Crawford has posted an analysis of english language open access journals in mathematics. http://walt.lishost.org/2015/06/the-open-access-landscape-17-mathematics/
statistics has the highest number of free journals outside the humanities and social sciences
there are more articles published in the fee journals
there is a relationship between the size of the journal and whether fees are charged, e.g. both prolific journals have APCs while 71% of medium-sized journals do not have APCs
article statistics show robust growth
journal statistics show a big jump in growth in recent years, both for fee and free journals
Comment: the correlation between size of journal and whether fees are charged is likely based in part on the need for more prolific journals to have paid staff, office space, etc.
This post is part of the OA article processing charges project.