David Crotty has published "Is it True that Most Open Access Journals Do Not Charge an APC? Sort of. It Depends." in The Scholarly Kitchen.
Here's an excerpt:
Most journals in the study ["72% and 41%: A Gold OA 2011-2014 Preview"] do not charge authors, but the majority of authors are choosing to publish in journals that do charge. 27% (2,365) of the journals studied required an APC and were responsible for 57% of the articles.