TOPIC: Free fee for a group or custom CB-field

On my site I have members and premium members. It should be free to registrate if you are a premium member. Is there a solution for this? I have JACL Plus with a group named Premium members, is it possible to let everyone in that group to registrate for free? I also have Community Builder, and maybe it's possible to make something out of there? Any ideas or other solutions to be able to do this?