I always wonder about this question. Do you mean cover costs or make an income? Running a forum is often a frustrating, maddening ordeal, not for the faint of heart or easily discouraged. The only real reason to do it is because its a topic you're passonate about. The money should be secondary, right?
On the other hand, it can be an expensive hobby, so monetization to help pay the bills might be important to some people. At the end of the day, it all comes back tot he same thing. Content is king. If your content is excellent, you will have high traffic which will either result in good monetization or the willingness of users to donate.