At the Matrix Partners' SaaS meetup , David Skok, a five-time entrepreneur turned General Partner at Matrix Partners, gave a few insights on SaaS economics. In his talk, David explained that SaaS businesses are fundamentally different from traditional businesses, and have their own model and metrics. His hour long presentation can be read in detail - the SaaS business model and metrics
http://www.slideshare.net/DavidSkok/the-saas-business-model-and-metrics