When you get it going it blows everything else away in value for money. We routinely have hundreds of containers on-the-go and as many concurrent builds as we need; this all runs on a large AWS spot instance at a cost of about $50 a month. Added bonus: our docker cache is always available.
Buildkite provide a CloudFormation stack, but we just opted to run the agents as containers via ECS to make the setup quicker and managing them easier.