That's a great question, hamids54. I think it will come down to how committed your members are to the topic and community, and what the benefits of the premium membership would be.
- Member badge/recognition as a supporter
- Relaxed posting rules in private area
- Reduced noise level in private area, more serious discussion
- What else can you come up with?
I don't know if your community will find those or other benefits attractive. A few cautions:
- The more content you have behind a pay wall, the less search traffic you will get. In most of the forums I know, the main source of new members is organic search traffic.
- Generally, only a small portion of active members will pay to play. If you could get 10% of your active people that would probably be good, but that's a guess.
- If your best members start talking in the private area, the quality of discussion in the rest of the community may go down. Probably not a big risk, but watch out for that.
Good luck, let us know if you try it!