Alan, I had a feeling you'd be asking over here. I was pretty sure the author got a flat fee from a book packager or Media Tie-In company.
However, remember that your case is a little different, in that while one party holds rights to the series concept, the actual characters, locations and storylines are owned by the authors individually as is outlined by their original contracts.
It's also a different because what you have is not really Media Tie-in (yet, remember I said yet!), but an entity unto itself - orginal fiction written by several different authors using one set of guidelines. They're not using characters and settings created by you; only guidelines/mythos and the series title were created by the parent company. Everything else is owned by them.
I would still ask a lawyer. There should be some percentage that's agreeable to everyone. I would guess somewhere in the range of 10%-15%. This is seperate from your own contributions as an author to the series.