It's hard to work out accurate numbers because users can have such drastically different usage patterns. Some users have signed up with one device and have only received a few notifications, others have many devices registered and receive dozens of notifications daily from chat plugins.
The API is free, though applications are limited to a reasonable number of messages per month. That was put into place out of fear that a large company would use the app/service as a dirt-cheap platform to send large amounts of messages every day to its employees (like routing/dispatching directions, etc.) and burden the service. And rightly so, since shortly after launching, I got an e-mail from a large US company looking to use it in such a way (they ultimately decided not to use it). If such customers do come along, their API fees will cover them being on segmented servers.