Bingo Cost for a hosting forum

How much does it really cost to run a relatively small* web forum (say 50-100 active users, and hosted in a "first world" country)? A few different examples have made me wonder. One forum collects US$ 3000/year in donations and advertizing, but still asks for more. In contrast, another forum's owner says about US$ 200/year will suffice. Other forums ask for nothing.


* Not sure how to define the "size" of a forum --is it the disc space required for old posts (and maybe lots of large images)? Or is it the data transfer, which may vary hugely depending not only the member activity but also on the amount of content/bloat and bots that keep requesting it?

Source.