[vonnik]

Market prices are determined by the intersection of supply and demand, so it is not just the buyers who choose. Google has some say over the supply. They may increase or decrease the supply based on behavior they sense more broadly in search.

It's amazing to see on HN that some of the times I am most right, I am also most downvoted. Life lessons...

[soared]

Well Google is the intermediary. They can (and undoubtedly do) directly affect supply, demand, and price. They can purchase ad space from a publisher for $0.45, mark it up x, and sell it. They can slow demand by temporarily (even for seconds at a time) by increasing cpc. They can lower supply by lower bids. They could in theory even pre-sell ads when supply is in excess.