Thanks for the link to your system. Is there a demo instance somewhere?
Yeah, emails are quite expensive. AWS itself charges $100 per 1 million plus additional infrastructure to manage subscriptions + email validation & dealing with bounces.
Therefore, email notifications are an optional feature. I hope in the future push notifications will be supported much better by the browsers so that it won't make sense to use emails.
It would be cool if you could add a "Deploy to Heroku" button or similar. Or at least screenshots. I think it would be nice to see how the UI looks like.
It seems very "expensive" to send emails.
Looks like a cute product, and the pricing mechanism is interesting. Good luck.
(I wrote some a simple self-hosted system, far less featureful, but it seems popular enough - http://github.com/skx/e-comments/ )