CEO interview: an insider's look at SaaS vs. on-premise software
The first key metric that’s important to a company like Carbonite is “cost of acquisition.” Since the average customer only pays about $50 per year, we must contain costs associated with acquiring new customers. Even small changes in this metric can dramatically change the company’s profitability.
“Retention,” or how many customers renew their subscription from one year to the next, is the other key metric. Managing retention requires a sophisticated stream of communications with the customer to ensure he or she is happy throughout the lifecycle.
via blogs.zdnet.com
Two of the most important metrics in a SaaS company. You keep cost of acquisition low (sell online = self-service) and your customers/users happy.

