An interesting perspective from Randall Stross; also check out his new book, Planet Google
GOOGLE controls about 70 percent of the search advertising market. Doesn’t that give it a monopolist’s ability to set prices as high as it wishes?
It does not. Google does not set the prices. Its advertisers do, bidding against one another for the amount they will pay when a user clicks on one of their ads. They do the same for ads on Yahoo and Microsoft search sites, too.