Is there a free version?
Yes, there are currently two options to try out Corona:
Corona Renderer for 3ds Max – Universal Installer
This is the commercial release of Corona Renderer for 3ds Max. It comes with a Universal Installer and can be activated in both FairSaaS and Box license modes. If you only want to try Corona, you can activate it in a 45 days unlimited demo mode. There are no resolution limitations or watermarks, and you can use it even for commercial purposes.
Get the demo version here: Corona Renderer Download Page
Alpha V6 for 3ds max is free forever. It does not have any time lock and it is a production-proven build. It is freely available without watermarks, resolution limitation, or any other limitations. The only problem is that it is is no longer maintained, so no bug fixes will be available.
Get the A6 version here: Corona Renderer Download Page