Thanks for writing this, I agree that the EC2 often dominates the AWS costs, but I tend to disagree with the statements that implied that the RIs are the best option for saving on EC2 and that spot instances are only for advanced use cases. With enough automation they can become viable for most of the use cases.
I actually wrote an open source tool that makes it easy to use the spot market by automatically replacing the on demand instances from your existing AutoScaling groups with compatible and almost identical spot instances, and it can be easily configured on existing infrastructure. It also allows you to keep a fraction of the group as on demand or RIs in order to reduce the risks of spot terminations, and will temporarily fall back to the on demand instances when scaling out or handling the spot instance terminations.
Please give it a try and let me know if you have any questions or feedback about it.