Unless you're on the business end of things and most of your new customers come from search engines - in which case optimizing your site, which offers the products people are actually searching for, to the top of the list (and aren't cheating with giant link-farms....). Good SEO has a lower CPA than just about any other kind of marketing.

Now, if we're talking about SEO where people are link-farming to spam pages just to get ad-hits and get ad revenue and other spammy behavior of no real value to anyone but the person doing the SEO.....i'm all for hte new classification.