You could sell links in your owner/admin signature line space. That worked on my forum. They paid $10 for one month. I've sold that space numerous times. Also, home page banner advertising at the top is very popular. Depending on how many monthly impression your site gets, you can earn well from that. Some high traffic forums charge over $100 a month for home page banner ads.
Yes, you can earn money with your forum... but not because of it's pagerank. While many won't agree with me on this, pagerank is really redundant and even google admitted that it isn't a major ranking factor anymore. You should focus on making your website more people because, as Beverly outlined, advertising revenue is your main source of income. If your niche makes it possible to offer premium content you can also go with that - many people are making thousands of dollars from their forum subscriptions.
PS: If you sell advertising on your site make sure to nofollow it or both your site and the advertiser will be hurt in seo terms. It is against Google's policy to sell paid ads and not noFollow them. As for how they find out if you are violating this policy... no idea, they are probably smart enough and they surely do.
Thank you for the information. I wonder how to set it 'nofollow'? Thank you. And if I set it 'nofollow', maybe people wouldn't like to make posts in my forum?