Did you notice the Display Network in your reporting and adwords interface. It is the new name for earlier content network. However, it is not just the change of name, and like every other change, google has imposed Ignorance tax with this too. Earlier, content network used to take the default bid of the adgroup in question to decide upon the max CPC of the adgroup, if you don't set the specific bid. However, its new incarnation, display network, will use avg. of all the adgroup's CPC. So, if you were in the habit of setting default bid for the content network, you might see unexpected CPC in your campaign. Most of us used to set up separate content network, and so we all were in the habit of leaving the field blank, so that default bid may be used as content bid. However, now, if you are continuing this practice, display bid won't be the default bid of the adgroup, rather it would be using an average of all your CPC bids. This average includes, in the order of priority:
Individual placement bid
Managed placements bid
So, the default bid is the lowest in order, and you should know, it does not equals auto in your adgroup. If you have set up managed placement bid and individual placement bids in your adgroup which with rare exception we set up at higher value would impact the auto bid. Hence, if you were thinking that default bid would be used if you leave the desplay network bid ad blank, you are wrong. If you haven't set it up, you should do it as soon as possible, so that you are not charged in excess of what you wanted.