As Waltham herself points out, the study in question is an interesting exploratory study which points to the need for more resesarch – a small sample of 8 journals, each self-selected by the participating publishers (who apparently all selected their flagship journals, quite possible the most expensive ones).

Quoting the average cost per article overlooks the wide difference in cost per page reported – for the online first copy, this ranges from $90 to $1,326 per page.

My analysis of the potential for humanities and social sciences to move forward towards OA, based on this study, can be found at http://poeticeconomics.blogspot.com/2009/09/humanities-and-social-sciences-thoughts.html. My comment on the wide differences in reported costs is at http://poeticeconomics.blogspot.com/2009/09/cost-per-page-for-hss-journals-varies.html

In Humanities and Social Sciences (and probably other fields as well) Gold OA publication fees may well be unsustainable today, prior to Green OA. But after universal Green OA self-archiving mandates by institutions and funders make the final refereed drafts of all journal articles freely accessible online, all bets are off, for then institutions’ subscriptions can be canceled, inessentials (print edition, PDF edition, archiving, preservation, access-provision) and their corresponding costs can be cut (offloading all archiving and access-provision onto the worldwide network of Green OA institutional repositories), journals can downsize to just providing peer review alone, its small remaining cost easily covered by institutions, per individual outgoing article, sustainably, out of just a small portion of their annual windfall savings from their subscription cancelations.
http://eprints.ecs.soton.ac.uk/15753/1/greenroad.html