Have you looked at what the equivalent would cost on alternative
systems? It probably depends on the mix of services you need, but for
me appengine is far less expensive.
Don't forget to factor in the cost of your time. With appengine, you
don't need to micro-manage email, database or web
It would probably be more cost effective to have a page people can
visit to get the daily summary, rather than using email.
I would also wonder why your pricing structure can't afford 3 cents a
month per user to cover the email expense. But without knowing what
the application is, I can't really
