Depends what you end up really wanted out of it.. right

A forum could earn a good amount of revenue if it has good content and if it generates a good amount of traffic. You just need to be patient and focus on creating high-quality content and have enough patience so that you could generate enough traffic. You could also add an article section to your forum and start publishing sponsored posts by various brands and companies. This would also help you generate a good sum of revenue.

My 40k member forum was making around $2k for a little bit a month. This was off of ads, marketplace sales (I took 20%), memberships and donations. I had around 500k visits a month and was listed first on google for about 10 keywords. So at least that much (Ad revenue was about $2k/year so it didn’t make nearly as much as you’d think)

Ads are minimal revenue for us, most is from user upgrade accounts. However in terms of making money yourself I think we do well with subs because we are non profit, ie any profit is donated to charity and members like that.