I've had them all along. Mostly for
ebay.
Most forums are free for users, but cost money to run, for hosting, software, etc. That's what the ads pay for.
I've been on a couple that were supported by the owner, but they were much smaller operations.
The baseball card one has ads, but they're smaller. I don't know if that can be chosen by the board owner or not. I suspect larger ads pay more.