Your podcast may have a target market. If you are a blues musician, do you want to advertise on a top 40 station that might have a very small amount of listeners who enjoy blues music, or would you like to advertise on a podcast that has 400 blues fans? It's not the number of the audience, it's the amount of passion the audience has for that topic (and host).
Lastly, if you advertise on radio you ad appears and immediately evaporates. I have a podcast that hasn't had a new episode in 2 years. Yet in September of 2011 that “dead” podcast had 2521 downloads (about 180 downloads per episode). If you had advertised on that podcast it would still be reaping benefits two years later, and radio can't even touch that idea.
Here at the site we have tutorials on making a media kit, as well as samples of other podcasters who have created a media kit. When you buy the book you get access to the information in the book, as well as any new items that have been discovered since its original publishing.