I really like looking at the Visitor Loyalty metric under Visitors in Google Analytics. Make a custom segment for just that display campaign and see how many return visits those ads created.
If this is the first time the visitor has visited the site the chances are slim that they are going to buy. But making something compelling enough to get them to come back should be looked at as a mini-win that leads that visitor down the conversion funnell. Look at the Visits With Conversions custom segment to get an idea of what the typical amount of visits to conversion is on your site. Let’s say its 4 visits, if your display campaign begins the visitor down that 4 visit funnel, that’s worth something.
This just goes to show the importance of giving the visitor some kind of hook on the landing page to invite them to interact with you again: you can teach them something, watch a video, engage in a conversation, subscribe to email, RSS or at least a cookie on their browser to send them a re-marketing message later.
When you look at display advertising this way, rather than only at direct response and conversions, you begin to ask the question of how much time and effort your business spends on pre-purchase transactions and how much you spend on trying to simply close the sale?