Haven't seen anybody charging for a guest comic, but who knows, maybe some do.
In my personal experience, authors reach out to peers through forum threads or social media, asking, yo anybody want to help me out with a guest art/comic?. And well volunteers make a comment saying that they're willing to help out a fellow in need =].
Some may take the chance for the extra exposure, no doubt about that.
In my case if I like the person's comic/art, I find the characters appealing or the author is my friend I gladly help them, no need for payment or any kind of trade, I do it out of pleasure really, I like to draw other peeps characters XD. The only logic condition I ask is that they credit my work.
And yes, to me is a lot of fun =].