According to a source, the contract is one of the largest ever for a non-fiction book, in order of magnitude only surpassed by Barack and Michelle Obama.
Bill Clinton is said to have received about as much as Spears for his "My Life" in 2001.
Spears is said to have wanted to write about her life since her sister published the book "Things I Should Have Said," in which she also tells the story of Britney Spears.
The pop star has criticized her sister harshly on social media and accuses her of trying to make money on her life.