EC2 Spot instances can be spun up much cheaper than your typical EC2 Instances. The drawback is that AWS can shut them down at any time when a customer willing to pay the standard rate needs them. This means you will need to code your workers to be resilient. Perhaps use transactions that will roll back if interrupted or otherwise find a way to track your jobs status so they can be resumed later. I cover some of this in my e-book 20 Things You Can Do Today To Save Money On AWS. You should be able to utilize this with ECS and AWS Batch as well.

It is quite a brilliant way to optimize their revenue while giving people a cheap option to purchase compute power that would otherwise be sitting idle and making nothing.