may be someone already said it but, what about a pay-per-listen option?
You wrote you're going to charge on some contents and, as far as I understand, there two options now:
1. a 'premium' content set of podcast with a monthly fee
2. a set of podcast on a specific topic that we can buy
My option is: you have a 'premium' content periodical (week? month?) podcast. If I"m interested in the content of a specific podcast, I buy it, otherwise I skip to the following issue.
P.S. It's a suggestion from my wife :) We were talking about this topic driving the car back from a week end on the beach ;)