On the first tab of your Banner Wizard is Opportunity Cost . That page will allow you to define what you consider the likely click-through rate will be and what your opportunity cost should be. Third - the ground rules. Different sales sites have different objectives. Some sites may have a complex objective and therefore need a balance to be struck between their different objectives.
Some sites have a very simple straightforward objective.
If you are seeking to establish a brand name or to reinforce an existing brand name then there is some value in exposure alone. You do not necessarily want the viewer to do anything. In this case exposures have a certain direct value for you. If you have a site that typically requires an ongoing dialogue or a number of visits before a sale is effected or indeed if you are a content site intent on building traffic then click throughs have a certain direct value for you. If you have a site that is selling a relatively low cost product, that probably makes it an impulse purchase.