There are different kinds of journals and so the costs and benefits can differ widely. Academic archival journals typically have volunteer editors whose salary is paid for by their employer rather than the publisher (usually a non-profit society such as ACM). These editors manage the peer-review and acceptance process. Once the papers have been accepted, they go into production and, depending upon the journal and its goals, they may be subject to simple editing for mark-up and obvious spelling/grammatical errors or they may be carefully edited to improve presentation, style, and content. Professional editing greatly enhances the quality and readability of every author's work. This is the traditional publications model used by professional societies. I have no idea what the relative costs are relative to a Wikipedia-style journal with a totally volunteer staff. What I do know is that the amount of human effort that goes into such a quality journal is significant and that no matter what the infrastructure is, someone has to put in that time, and their efforts should be compensated in some fashion.