My guess is that Fox looked at the books and the viewership and decided that Sarah was being paid too much, and offered her less and she quit in a huff, like the diva she is. That would be consistent with her modus operandi.
Odds are that if she has a good financial adviser, she can invest the money she made on her two awful books and her speaking engagements and retire in comfort, probably in some place warmer than Wasilla. I hear she bought a house in Arizona.
I wonder who financed the rental of her bus during the first part of the primaries, and what was her purpose. I imagine that she was hoping for several hundred thousand 'bagger buddies like Kramer and sirs would rise up and support her for president, but discovered that her 15 minutes of fame had expired.