We've thought about it, but Cushion is incredibly lightweight and doesn't need to be used constantly, so our load is very low. We've only needed 2 dynos since we launched and we're nowhere close to needing more than that. As much as I'd like to lower the cost, they truly take so much weight off our shoulders. I gladly pay what we're paying now, so it's not an issue for us.
Have you looked into Appengine Flex. Appengine flex is a PaaS like heroku, and will scale up any docker container. Its a great solution for your early days, and when you do need to move to a 1000+ node cluster its easy to switch from Appengine Flex to kubernetes. You also might get a lot of free startup credits https://cloud.google.com/developers/startups/