Media Buy Negotiations - How Much Should I Offer?

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.

Does the forum already have ads on it? If so, Adsense ones or just single website ones?

It has two text ads and they aren't adsense.

Usually, it is not the Advertiser who decides how much to pay to the forum owner, its the other way round. I have also dealt with forum admins in the past regarding ad space buying, and they already had a figure in mind which they offered me. So to start off you can approach them by writing a simple mail or by using the online form to the admin. They would either respond with a pre-set price or if you are lucky they wouldn't know much about space selling and you might be able to get a cheap deal.