Gary's point is right - is it really necessary? Often the opportunity (for the link and exposure) is pretty valuable by itself.
More importantly, trying to manage a revenue-sharing program can be very difficult - for example, say a site visitor comes to a site to read a particular article by a guest blogger, then while they are still on the site they wind up reading several more articles by other authors, and at some point they decide to click on an ad and buy something.
How do you split that revenue? Does it go to the author whose page first attracted them to the site? The authors on whose page the ad they clicked on was located? Try to figure out how many pages and whose pages the visitor read and split it amongst all those authors?
I like the idea of the other blog that Rebecca guest blogs for uses - allowing guest bloggers to link to a product or service that they are selling.....one that pays them directly. That would make it both attractive to guest bloggers, and very hands-off to manage. Hmmmm..... I may have to consider something like that for TWE!!