Andrew Gray has published "Considering Non-Open Access Publication Charges in the 'Total Cost of Publication'" in Publications.
Here's an excerpt:
Recent research has tried to calculate the "total cost of publication" in the British academic sector, bringing together the costs of journal subscriptions, the article processing charges (APCs) paid to publish open-access content, and the indirect costs of handling open-access mandates. This study adds an estimate for the other publication charges (predominantly page and colour charges) currently paid by research institutions, a significant element which has been neglected by recent studies. When these charges are included in the calculation, the total cost to institutions as of 2013/14 is around 18.5% over and above the cost of journal subscriptions—11% from APCs, 5.5% from indirect costs, and 2% from other publication charges.