The initial costs of my forums were pricey (XF software license, add-on’s, themes, domains, hosting) but now that everything is established, just maintaining is relatively inexpensive.
Renewing my XF license, add-on licenses and themes licenses is optional and doesn’t need to be done unless I choose to do so.
So that just leaves the domain and hosting renewal (which are mandatory) and those aren’t much money. The way I have them set up, I only have to pay those once every 2 years.
I've typically invested a few hundred bucks in my sites to set them up and cover initial advertising. I found a good way to monetize was via VIP programs and programmatic advertising on blog type landing pages -- not so easy on forums themselves though.
Though I love XF, I also love phpBB so I'm quite lucky in that a number of my sites haven't cost anything in software licenses at least.
The domain and hosting usually alongside perhaps one or so paid addons/plugins. As some said, forums can go either way nowadays. Some are successful, some just stay small. I also will do paid posters with forum currency if they accept it. I prefer that over IRL money tbh.
I have invested money into my forums but I will only invest money into things for the forum if I feel it is worth it and it will pay off in the long run. I start off small and then increase as I know I need to.
