Is there somebody with an Ad Sense account who can tell us what the cost for this is approx per month?
I doubt it will cost much for them. Google targets the advertising. Doing so, those that are re-visiting bitcoin sites will be profiled. Assume that is 10,000 bitcoin miners and users of bitcoin. Targeted advertisement is hi quality, and can be charged more, but let's keep it simple and assume a cheap 1 cent / banner impression (i.e. without a click on the ad).
10,000 x $0.01 = $100
Disregard forum ads. Let's say that many ads are dispensed every two days. So, $50/day for the adsense ads per day to be displayed. More if there is significant clicking. Assume 4% click on an ad (which is pretty high percentage), and 25 cent rate for the click. You get:
400/2 click thrus x $0.25 = $50 more per day.
So, my left field guess: ($100 to $200 per day) or a median guess of $150 per day for the ad sense ads alone. I don't know what the forum(s) charge for ad space.