I have been wanting to bug my guys here to make it so that the YPN ads interchange with AdSense ads. But they are busy and I don't want to bug them with my little games. So instead, I found a thread with someone who actually did it himself. Message number nine, in this WebmasterWorld thread says;
I run Adsense and YPN on the homepage of my main site interchangeably to test which is better (equal pageviews at the end of the day, same weights). Both ad networks are both on target, although Adsense ads show more "less sophisticated" adverts (no brand name, mostly mom and pop operations); while YPN shows brand names. Google's Adsense gives me double digit CTR, while YPN only gives me 1/10 of Adsense's CTR. At the end of the day, even if YPN gives me higher earnings per click, Google gives me better revenues. It seems that G's targeting is designed to show what ads fit the page AND what ads are most likely to get click (probably due to some historical data of my target audience or whatnot). On the other hand, YPN only shows what ads can possibly be targeted to the content, but not really designed to generate high click thrus.
And to be honest, that was my thinking from day one. The CPC seems high on YPN but the CTR seems lower, due to the relevancy issues with YPN. I find the post I quoted above, to be very helpful and I hope to find more.