PeerJ is different, though, in that you pay through lifetime memberships rather than a new fee every time you want to publish an article. Their fees are also substantially lower than those of the traditional publishers.
No question. I support PeerJ to the fullest extent and the price is more than fair. (It's actually unfair, honestly, it's far too low.)
But there is a fee. And it's important for supporters to be honest so it doesn't appear one-sided, so, here I am.
My company has to pay to publish in our preferred journal of open access (PeerJ). I gladly pay for lifetime memberships for all of our authors, which is pittance and affords lifetime publication rights with no additional fees.
But we do pay. Most OA journals do have fee waivers and are very liberal with them, though.