In fact for those unfamiliar, it might surprise that the common model is to "undercharge" systematically based on something called second-price auction. This includes ad systems like Google AdWords and Yahoo Gemini. In a nutshell, the highest bidder wins, but pays only the bid of the runner-up. See https://en.wikipedia.org/wiki/... for some additional details. Needless to say, there are other complexities including reserve price calculations, so there are accidental screw-ups. Advertiser discounts like those mentioned in the article are quite common.