I do understand that a self sufficient or standalone android application
can be bought/sold for one time price.

But what are various ways a user can be charged if an android application
was a service based application, meaning every time user playing a new game
would fetch some data from a service.

It seems to me that the pricing model should be on usage bases,
be it periodically (monthly/yearly) or be it per use.

As an hypothetical example, if using gmail on a g1 has a price tag, what options would
the Google team may have, in terms of making users pay?
Can someone please throw some light on the subject?