If there is a website with no other advertising then how does advertising cost per advertiser change as the number of advertisers go up if all ads are the same shape, size and location? EX: Daringfireball.net has one ad on the side that he gets $4800 for (according to the deck). If he were to increase that number to 2,3,4 or 5 what would the value of that single ad be?
Advertising cost mostly has to do with the number of click-thrus expected, therefore traffic and the quality of that traffic. Depending on the site the price might not change very much. My guess for daringball is that if the site owner offered 2 ads the price would be about the same, if they went up to 5 or more the price might start to drop but not in direct relation to the number of ads. Maybe this site owner just sells that one ad to cover costs, and does not really believe in selling advertising or maybe he has more ads for sale but only 1 is taken right now?