Me and my twin brother have had it as a passive income thingie for years. After a failed attempt at selling it (SaaS metrics are NOT easy) we met a guy who saw some potential and we're finalizing paperwork with our lawyer to create a Swedish limited company right now with him as co-founder. We will try to take the plunge and get it running as a full time company within a couple of years!
Achieving a lifestyle passive income project is surprisingly hard...
Thanks for giving me a simple place to go after Google Reader shutdown!
We're focusing on building a great product and a scalable business. Making money from our users should be off something that benefits them as well, like a pro service with more features.
Then, the user, in fact, pays only for using the back-end API. The front-end, of course, is like a website, """open-source""".
Then we have additional costs of http://chartmogul.com (love. required to know your SaaS metrics) http://intercom.io/ (love love love, awesome product).
Edit: Oh, and after a really bad downtime we're paying for pingdom and statuspage.io. So our total expenses are now probably around 1000 USD per month. Braintree and PayPal also take around 5%-19% of each payment.