It does depend very much on the size of the community you plan to serve.
There are several packages on offer, ranging from cheap-and-cheerful to all-singing-and-dancing. The one I admin on uses phpBB, which is pretty easy to operate - but you do need to get it installed and set up.
Some webhosting companies offer BBS software as part of the package. A bit of googling will find you one. You may even find that your own ISP offers it!
It looks like you'll pay £5 or so a month for the "entry level" with all set up for you. More googling than I just did might get it for less.