I don't know why PLOS charges more. The point I'm trying to make is that - just because there's an OA journal out there which charges $100 per article; it doesn't mean that the costs are justified.
Do you have any compelling evidence that the costs for OA journals is one or two orders of magnitude less than everyone claims they are? (See e.g. http://www.nature.com/news/open-access-the-true-cost-of-scie... which also shows a wide variety in publishing costs.)
