Well, there's some price point at which if everyone paid the OA fee (effectively making your entire journal open access) then the revenue would about equal the

Well, there's some price point at which if everyone paid the OA fee (effectively making your entire journal open access) then the revenue would about equal the revenue you get from traditional paywall subscriptions for that journal, and the publisher wouldn't care one way or the other. In reality it's almost impossible these days to figure out what the paywall revenue from a single journal title is anymore, since everything is bundled in huge packages. But yeah, more or less the fees are based on what the publishers think authors will pay based on the prestige offered by the journal. There are people trying to change the typical price point of OA publishing (PeerJ's $99/author fee comes to mind), but there's A LOT of intertia in academia.