Every single Podcast in iTunes is FREE. There is no way in iTunes to charge for a podcast.
What you need to do if you want to have a premium offering is keep some episodes for Free in iTunes and then use a 3rd party to help you manage a premium offering that is spearate from iTunes. This is what Marc Maron and Adam Carolla do. The both use the Mylibsyn premium offering.
You do need to host your files with libsyn for this to work.
Let me know if you have any questions on this.
rob at libsyn dot com