Some of our builds are running on CircleCI but have been trying out drone for a more container focused build. Been enjoying the flexibility and increased speed from custom build images which are trivial to make.
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.
Another similar option we tried is AWS CodeBuild which has per-minute billing and provisions the build machines for you. However, it's very bare-bones and because you always get a from-scratch instance for each build you have to distribute your docker cache which is not ideal.
We actually use a combination of on-demand and spot container instances in production to keep costs down; we have some logic to provision more on-demand instances in case of multiple spot outages.
[0] https://aws.amazon.com/blogs/compute/powering-your-amazon-ec...