According to an Us Magazine article from December 2005, here are some of the details of their pre-nup:

• K-Fed won't touch a penny of Britney's money. All the millions belonging to Britney will stay that way.
• The mansion will be divided 50/50. Any gift given worth over $10,000 will go back to the original purchaser. This includes a custom-built motorcycle Britney gave K-Fed for Christmas 2 years ago.
• She will still support him for a little while longer. Britney will pay Kevin $30,000 a month for the period of time that is half the length of their marriage. Meaning he'll be taken care of until at least a year from now.
• Spears won't pay child support. Federline is still responsible for supporting his other two children with previous baby momma Shar Jackson.