"the two most substantial of which, manuscript writing and reviewing, are provided free of charge by the scholarly community [20]."

That peer review is 'free of charge' is a common fallacy, and it would help if academics who write about peer review could get their heads around the economics. Although editors and reviewers generally are not compensated, the editorial office staff are paid professionals. Their workload increases with submissions, and includes checking in new articles, sending reviewer requests, chasing reviewers for their comments, checking editorial decisions and performing final checks on accepted papers. One might be tempted to think that each of these tasks is trivial, but the Pareto effect comes into play: 20% of the articles take 80% of the effort.

Journals with higher rejection rates thus have higher peer review costs when they are calculated per accepted article, which is probably why OA fees are much higher at journals with low acceptance rates.