As far as I know PLOS editors are unpaid just like reviewers. People keep saying that there are costs to a scientific journal, but don't say what those costs ar

As far as I know PLOS editors are unpaid just like reviewers. People keep saying that there are costs to a scientific journal, but don't say what those costs are exactly. Sure, the costs are nonzero like hosting, but what costs more than say $1 per article?