I personally think its a supply and demand thing.

The more available ad clicks vs the desired ad clicks on a ppc network the lower the price.

I personally (As a publisher) wish google would stop or at least slow down in allowing new sites and also crack down on "google sites" That are there exclusivly to get ad clicks.

I also sometimes think that the ads are TOO relevent. So some people are complaining that yahoo isn't, but i think that is a good thing.

One of my sites is free image hosting, so I don't want ads for other free image hosting sites. But, image manipulation tools would be very good. Funny how i get plenty of picasa ads, but none for photoshop or fireworks and so on. Flash drives would prolly be good, digital cameras, even camera phones. I see none of those ads with google.

Now granted I stuck some yahoo code on the main page of a free forum hosting site and got ads for log cabins, but that is another story...