Heather Morrison et al. have published "Open Access Article Processing Charges: DOAJ Survey May 2014" in Publications.
Here's an excerpt:
As of May 2014, the Directory of Open Access Journals (DOAJ) listed close to ten thousand fully open access, peer reviewed, scholarly journals. Most of these journals do not charge article processing charges (APCs). This article reports the results of a survey of the 2567 journals, or 26% of journals listed in DOAJ, that do have APCs based on a sample of 1432 of these journals. Results indicate a volatile sector that would make future APCs difficult to predict for budgeting purposes. DOAJ and publisher title lists often did not closely match. A number of journals were found on examination not to have APCs. A wide range of publication costs was found for every publisher type. The average (mean) APC of $964 contrasts with a mode of $0. At least 61% of publishers using APCs are commercial in nature, while many publishers are of unknown types. The vast majority of journals charging APCs (80%) were found to offer one or more variations on pricing, such as discounts for authors from mid to low income countries, differential pricing based on article type, institutional or society membership, and/or optional charges for extras such as English language editing services or fast track of articles. The complexity and volatility of this publishing landscape is discussed.