Didn't he get an inker pretty recently? Also, the prices for his originals are really high, mostly due to the popularity of the show/book.
As for Rat Queens, I imagine he's still co-creator, meaning % cut for anything beyond the comic forever and for the issues he worked on. The replacement will get either a page rate or a % of profits for the issues they work on but no real ownership in the property.
This is going to be tough for the book though since artists styles differ so drastically on how they present the story/scenes etc...