Subscription Fees for Forums [Discussion]

I've been thinking of creating a forum with subscription fees, after seeing how similar forums have greatly succeeded. What I want to know is how paid subscription based forums continue to make money and what makes them stay popular with all the other alternatives around?

For example, why signup on the SEOBlackHat forums for $100 a month when I can find almost the same info on other SEO forums for free? Why would someone pay to have access to discussions and information that is available elsewhere on the internet? Usually people don't, but I guess the trick to making people pay subscriptions for forums is by offering content or communities they can't find elsewhere.

What are the disadvantages of a paid forum? How can you create an "IN" community that people want to join? How do you keep members subscribed? Is it wrong to do this? For a forum that is just starting out, do you start with no fees and build up a userbase first?
Any ideas are welcome

BTW: I'm not talking smack about SEOBlackhat; it was just the only paid forum that came to mind.

 

Think hype.

 

I don't think you will succeed until you have 10,000 or more visitors per day.

 

Not true Ringboxer.... If your selling a really good product or subscription, then small amount of visitors will buy it.

it's about quality.