Setting up a forum is very cheap. You can get a forum up and running on the internet for around $58.99 a year.
That cost includes around $4.00 per month for a shared web hosting account and $10.99 a year for a domain.
So the forum costs less than $100.00 per year to run. That is for the start of the forum.
Once you get your forum very active then your cost might go up when you move your forum to a vps or dedicated server.
There's good free forum software such as mybb that won't cost any license fees to use.
A lot of people use V bulletin forum software which costs money but V bulletin forum software has went downhill.
I recommend just using mybb.
The easy part is getting the forum online. But the hard part will be getting the forum active with new members.
You might have to create some fake users on your forum and talk to yourself for awhile. It does work.
It shows that the forum is active and people are talking to each other but in reality it's only you.
No one wants to sign up for a dead forum with no posts and only one person talking to their self's.