To get an idea of relative savings in reserved instance prices compared to ondemand ones , you can try http://promptcloud.com/ec2-ondemand-vs-reserved-instance-pri... . It was HN couple of months back.

For SaaS startups I found this post by David Skok to be very informative and comprehensive http://www.forentrepreneurs.com/saas-metrics/ . It is a bit old now but is still very relevant. Author talks about David Skok in the article too.

Edit: Added ref to David Skok