The cost of publishing an article consists essentially of content production costs (research), review-and-editorial oversight, article preparation, display (in the electronic world), and archiving.
Content creation, the research itself and manuscript preparation, is assumed to be a part of everyday academic activities and is covered by salary and grants. So too is the review process and much of the editorial oversight - all part of the daily give-and-take of academic life.
The only real costs - out-of-pocket costs - are such items as copy editing, file conversion to PDF, HTML or XML, and the cost of maintaining the electronic publishing platform. Rapid evolution in and simplification of technology have driven many of these cost elements down dramatically.
As one simple example, documents created in Microsoft Word or Open Office can be converted automatically to PDF as a final step in document processing - and at virtually no cost. There are services available globally to convert documents to structured-and-tagged XML or HTML(for submission to specific electronic archives) for less than $1 a page. A 5-page article could be available in both PDF and HTML for $4.50. Copy editing is in a state of flux as well, with many journals asking their authors to submit final versions fully copy edited. Should a journal wish to handle the process itself, the cost per article should be in the $40-60 range after the journal editor and the author have polished the content.
Add to this a free or minimal cost publish-and-display environment, and the per-article cost should be in the $10-$100 range rather than $4000 as commercial publishers reported - two orders of magnitude less.
This 2008 article from the Journal of Electronic Publishing provides a more detailed discussion about 'real' costs.
There are, of course, a small number of scholarly journals that depend on their publications to generate revenue for a society or conference, and the facilitated publishing model that Scholarly Exchange offers, with optional advertising sold by the journal itself, supports that opportunity.