The key here is including the depreciation of the audience. Bad advertisements (to the viewer) turn people away from a particular channel, so those advertisers should pay the cost of the lost audience in addition to the standard rates, while 'good' advertisements should get a discount for being relevant/interesting to the audience, and therefor retaining them. (Of course the actual quality of an ad might be subjective, though I'm sure clever media companies will learn how many viewers leave at each ad.)