AWS pricing is crazy complicated. I gave a presentation on this a few years ago (https://speakerdeck.com/jpsingleton/aws-pricing). It may be a bit out of date by now but I could update it and write it up as a blog post if people are interested?
It's not easy initially but things like http://www.ec2instances.info/ definitely help.
I agree strongly with this. I've had clients before who would refuse to make the move up from VPSs despite numerous advantages because the AWS product descriptions and prices were incomprehensible to them.