Flynn is entirely open source and BSD-licensed (https://github.com/flynn/flynn). You can run Flynn on any infrastructure without paying us anything.
The price you reference is for our Managed Flynn product, where we act as your ops team, operating a Flynn cluster for you and providing hands-on support for your apps and databases in production.
I know it's probably not as feature-complete as those other PaaS's, but it would be great to get it at a glance.
Many people who choose Flynn are not directly comparing us to hosted platforms, as they want more control over their infrastructure (where they run it, lock-in avoidance, etc).
As far as individual features go, we are closest to Heroku (and maintain buildpack/runtime compatibility). However there are differences including our support for other databases (MySQL and MongoDB in addition to Postgres and Redis) and many other smaller things like HTTP/2 support.