> And Google dictates the rules of the auction marketplace.
Every company "dictates the rules" on which its sells product. Pricing power -- the key test in antitrust for a monopoly -- requires that they be able to change those rules so as to raise prices within some range without business moving to alternatives; in the case of AdWords, it would mean that Google could make it more expensive for the same results without any net migration to any other advertising vendor.
Is that the case? I don't see compelling evidence either way.
It's an auction model. The buyer literally sets the price.