Considering that m3.large (7.5 GB of RAM vs. PX 6GB) instances, on demand, are $0.14/hour, it's probably much more than 50%. (I'm sure they are using reserve pricing to reduce the price by 50-70%)
Cloud66 gives you most of the convenience of Heroku and lets you use your own cloud provider (AWS, Digital Ocean, etc)
Still pretty expensive compared to taking the DIY approach on AWS, but I appreciate the price break here.
I imagine their markup on top of AWS is still something crazy like 50% or more?