I think provided you have access to the right kind of server hosting, or own and can manage same for yourself, the rest is relatively cheap. I appreciate relatively cheap is not quantifiable.
You need to look at hosting companies that provide this kind of interactive facilty and look at what they are charging. Many sell templates for forums as part of their hosting deal.
Of course, the cost in AB's case and many others will be offset by the advertising deals and this is a route that one could explore if starting out.