Paid based groups mostly mean you get an upgraded account and it helps the site as it pays them to keep the forum going, supports them. I allow subscriptions for anyone, doesn't matter if you are new or old.
You can offer paid subscription when your community is really huge and you offer useful and interesting information, or when you want funds and ask the users to support the community through a paid subscription.
I think sponsor memberships for people who donate to the community is reasonable. They get a nice donation badge or different colored username that shows their dedication.