DoubleClick Ad Exchange uses a second price auction model:
If Preferred Deals or Private Auctions are in play for an ad unit, they are taken into consideration before the impression goes to the open auction.
In the open auction, Ad Exchange matches buyers' targeting with sellers' inventory and seeks the highest bid.
The buyer with the highest net bid wins.
If two or more buyers bid the same amount for an impression, the winner is selected randomly.
The winning buyer pays the higher of the second-highest bid price and the ad unit's minimum CPM.