John Lee Dumas who runs EntrepreneurOnFire is very transparent with his monthly income and costs.

His latest monthly income report:

http://www.entrepreneuronfire.com/income15/

And more info on sponsorships:

http://www.entrepreneuronfire.com/sponsors/

According to the latter link he charges $18-25 per 1000 listeners. Should be noted that he produces daily podcasts. The CPM should really depend on your niche though.