In exchange, we get more exposure

Hmm ... Castify doesn't have any other content (and thus, no subscribers) yet. They're literally brand new; the castify.co domain was registered last month. They don't have an audience yet to which to expose or promote LW.

For me, this whole exercise pattern-matches much closer to "a tech startup promoting itself in a known technophilic community" than to "an effort seriously designed to maximize accessibility and promotion of the content" or a crowdsourcing ripoff of the sort David Gerard is describing. (The Wikitravel case is pretty icky, but it doesn't resemble this case much at all.)

Not that an act can't be both a promotion and an accessibility effort; and it's pretty low-risk for LW, but on the other hand —

— how much do you usually charge for endorsements?

I see no reason to imagine that audiobooks would ever come to exist otherwise.

Ever? That's pretty strong. Even conditioned on the existence of audiobooks of HPMOR — which cannot be commercially done?