David J. Solomon and Bo-Christer Björk have self-archived "A Study of Open Access Journals Using Article Processing Charges".
Here's an excerpt:
Article Processing Charges (APCs) are a central mechanism for funding Open Access (OA) scholarly publishing. We studied the APCs charged and article volumes of journals that were listed in the Directory of Open Access Journals as charging APCs. These included 1,370 journals that published 100,697 articles in 2010. The average APC was 906 US Dollars (USD) calculated over journals and 904 US Dollars USD calculated over articles. The price range varied between 8 and 3,900 USD, with the lowest prices charged by journals published in developing countries and the highest by journals with high impact factors from major international publishers. Journals in Biomedicine represent 59% of the sample and 58% of the total article volume. They also had the highest APCs of any discipline. Professionally published journals, both for profit and nonprofit had substantially higher APCs than society, university or scholar/researcher published journals. These price estimates are lower than some previous studies of OA publishing and much lower than is generally charged by subscription publishers making individual articles open access in what are termed hybrid journals.