I am thinking of approaching approaching a specific forum to buy advertising space for a banner. The forum in question has about 2-4k users online at any one time and I think it would be a great source of relevant traffic for my niche. I was wondering if anyone could give me a ballpark figure about how much I should be offering the site owners to run my ads. I have never approached a site in this way before, having always bought my traffic from facebook or google. If anyone could offer any relevant advice it would be appreciated.