The 'Toxic' singer eventually agreed to a $15 million payment for one season of the US version of the Simon Cowell-fronted talent show, $3 million more than the 'On The Floor' hitmaker gained for her first season on the rival programme.
A source told TMZ.com: ''Britney's bottom line was no less than $13 million.''
However it is also believed she initially asked for as much as $20 million, before eventually settling on the final sum.
The deal is said to have made bosses on 'American Idol' particularly nervous as they think Jennifer now may up her demands for the show despite ratings falling in the last season.
It is believed 'X Factor' bosses were so desperate to sign up Britney for the second season of the show they gave her permission to do whatever she wanted, which she did at her first taping of the show in Austin, Texas when she walked out during four auditions.
A source said: ''Simon Cowell is a smart and shrewd businessman who knew Britney could make his show and he was willing to give her just about anything to come on board - and he did.''