Dear Miss Snark,
Do literary agents get a smaller percentage from established authors (e.g. Grisham, Rowling) who are pretty much sure to be successful with whatever they produce? How about not-so-well-known authors who come up with a huge blockbuster like Dan Brown? Do the agents ride the gravy train along with the authors or do they get chiseled on their percentage when the going gets good?
Some very very big name authors (like Bill Clinton for example) don't have literary agents, they have a lawyer who negotiates for them and gets paid by the hour. MUCH cheaper, even at DC law firm billing rates.
Commissions are a matter of contract between agent and author and contracts can be amended pretty much any time the parties decide. Of course, they have to agree or it ends up in litigation.
If one of my clients becomes the next Dan Brown and makes one gazillion dollars, my contract with him says "fork over the 15% now and forevermore", and his contract with the publisher says "Miss Snark is the agent, send the dough to her".
If the client slithers over and says 'hey honey, time to get off the gravy train', we'd have a little discussion that would involve those crocodiles mentioned earlier today.
Frankly, I don't worry about this much. My clients tend to hover on the honorable side of the spectrum and most of them value the work I do. This may be because this is a smaller agency and I deal with my clients face to face (ok electron to electron heaven forefend they'd actually yanno expect to visit or anything) so there is a real relationship in place.
An interesting statistic is that a significant number of medical malpractice suits could have been avoided if the doctor had just spent more time listening to the patient and answering questions. I forget what the exact number was but it was in the 30% ballpark I think.
When relationships start to fall apart, and people start yammering about renegotiation, and reaching for the Bar Association rolodex, one of the smartest things you can do is talk face to face and really really listen.
Everything is negotiable, but not everything is agreeable.