Fredrick Marckini looks out how paid inclusion on Yahoo can bring in traffic more cheaply than paid placement through Yahoo-owned Overture, in some cases: Paid Inclusion Beats PPC in Many Markets, from ClickZ.
Of course, it's also cheaper because the traffic is less predictable. You don't know if you'll actually show up for a term. If you do, you could be anywhere in the listings, rather than near the top. That potentially mean less traffic than with paid placement.
On the flipside, paid inclusion ironically is a paid route into the free or "natural search" results that searches are more likely to click on, as the article notes. That could mean more traffic for you, in some cases.
For the many issues paid inclusion raises, see my past article: Going Beyond FTC Paid Inclusion Disclosure Guidelines.