I wrote an entry a while ago about why I think podcast advertising should be worth more than main stream media advertising. Without rewriting the entry, my basic premise is that although podcast audiences are typically smaller than, say radio audiences, they are more interested and better targetted. There's a good article on AdRants that talks about this same idea:
Mostly, the big hurdle for the pocast medium is getting advertisers to understand there's nothing wrong with small audiences. The industry has already bought into the notion of nice audiences. Podcasts simply extend that notion exponentially. And as Tim Borquin said in answer to an audience member who said he would not be able to get his client interested in such a small audience, if an IT advertiser could advertise on a podcast with only three people and those three people were Bill Gates, Steve Jobs and Larry Ellison, the concern would be put to rest. And that, my friends, is what podcasting can deliver.
While that might be an extreme example, the point is clear. Unlike radio, podcasting isn't push media. Some people will try to tell you that it is, but because acquiring the media files requires positive action on the user's part, it isn't. Podcasts are pull media and therefore the audience is almost certainly more interested and engaged than traditional media "consumers". Hence, they're worth more, no?