To get a feel for the value of CPA, I examined the last 50 campaigns we ran on a CPA basis. I then took all of these campaigns and turned them into CPM campaigns by determining the number of impressions it took to reach the target number of actions. This category included making a purchase, filling out a credit card application, or completing a multipage registration form. CPA can be an effective model if the campaign includes a worthwhile, relevant offer that's easy to sign up for. Be leery of advertisers wanting to run banners just on a CPA basis.
Banners provide branding and image beyond their CPA value, and advertisers understand that.
If they get a publisher to accept a CPA deal on a banner, they usually will get more than they're paying for. Publishers who are very selective as to what CPA campaigns they will accept should realize money can be made on them. If you wouldn't, then others probably wouldn't either. Make sure the link on the ad goes directly to the action page and not to a home page. It's all about making the process simple for the audience.