Regarding cost, I am just going to clarify here that “push notifications” is at the moment an “exclusive” feature for all our customers in all hosting tiers.
Per our discussion, once we get the Android version of the notifier done we can add it as a bullet point to our “buy” page which combined with ingoing and outgoing mail support and spam handling and excellent performance are making our hosting plans extremely compelling.
I replied here about what externally hosted Discourse communities can do.