Ithaka S+R has released The Costs of Publishing Monographs: Toward a Transparent Methodology .
Here's an excerpt:
While there have been numerous efforts to understand the costs of publishing a scholarly monograph, this study is unique in that we worked with an advisory group of university press publishers to identify all of the cost components in scholarly monographic publishing and to work with a wide variety of university presses to calculate their costs of each of those components in a bottom-up fashion.