I'm confused. Are you talking about mixing/sound design or composing? It varies depending on the project, of course, but here in Atlanta, it varies from $175-350/hr for straight-up audio post. $450-800 sounds more like online rates. No one would pay that rate for audio post here, unless there were some very specific and unique criteria. Of course, composing is different and absolutely varies. I've only taken it on a whole-project basis and never by the hour. Anyway, just a little confused on specifically what you were asking. Forgive me if I missed the boat.
-S