I listen to many podcasts. I sometimes wonder if guests get paid to appear in shows like on Leo Laporte's TWiT and John C. Dvorak's CrankyGeek. Like how much? Maybe not the one off guests. What about co-hosts like Amber McArthur on Twit's Net@Night?
There are a million different podcasts out there so I can only speak to the ones in the spaces I've been involved with. Guests don't receive compensation. Typically they're selling something - representing a company, selling books, pimping their own podcast. Some aspire to be seen as experts or influencers in a particular space so make the rounds based on building that up.
Co-hosts are a different situation. There has to be some exchange of value for their time/expertise.