As far as I know there are no podcasts that you can subscribe to in iTunes for a fee. People like Marc Maron sell individual episodes for a fee in iTunes I've helped people do this for year, but using the same path as musicians. There are tow companies at the fore front of sellign your podcast in iTunes. These are CDBaby and Tune Core. Here are some comparison notes:
1. Both charge $49 to get set up.
2. CDBaby charges no annual fee, but takes 9% of a digital sale. Tunecore has a $49 yearly cost, but takes no profit from you. Doing ths math ($49 / .09) you would have to sell more than 544 items to have tunecore be a better deal.
3. CDBaby has 60 digital retailers (iTunes Amazon, etc). Tunecore has 27
4. CDbaby has tools to help you sell your digital files on Facebook and other tools.
Another Way To Sell Your Podcast Episodes
While you can always benefit from the traffic of iTunes, don't forget you can sell directly from your website using PayPal and a tool like Magic Members (where you could sell individual tracks, or a compilation download). Magic Members is a one time fee of $97.