I suspect prices for real advertisers won't change much. But this more efficient market will have the same effect on advertising secondary markets as electronic trading had on stock markets.
At one time individual brokers in various geographic localities could interpose themselves between investors and the NYSE, and take a significant cut of every stock sale. But electronic trading cut the cutters out, and made investing more profitable for the people who matter -- the investors.
Google's continually trying to make their market more efficient -- so people who used to take large advantage of pricing inconsistencies will find the pickings slimmer and slimmer--they'll get squeezed out because advertisers can count on the market to give the best price. And this is just another step in the process.
Note that it has nothing to do with competition--Google's competition is other genuine market makers, not the arbitragers that parasite every genuine market maker. But the other real market makers will have to keep up with technology, or die. (Like the former search engines that couldn't match Google's anti-spamming technology...) So the end result of BETTER competition may well be FEWER competitors.
It's always a trade-off. Society can't afford too many competitors, any more than it can afford too few. I suspect consumers think advertisers are a glut on the market right now, and would appreciate less investment money channeled into advertising and more money channeled into creating content worth advertising.