I don't think the cost difference comes from the type of review, or at least not entirely. Consider that PLOS ONE, because it doesn't review for importance, ha

I don't think the cost difference comes from the type of review, or at least not entirely. Consider that PLOS ONE, because it doesn't review for importance, has a much higher acceptance rate. Only accepted papers pay processing charges, so each accepted article has to subsidize the review and rejection of all the rejected papers. In PLOS Biology or PLOS Medicine, with a low acceptance rate, each accepted article has to subsidize review of numerous other articles, driving the cost per published article way up.