The 'Toxic' singer eventually agreed to a USD 15 million payment for one season of the US version of the Simon Cowell-fronted talent show, USD 3 million more than the 'On The Floor' hitmaker gained for her first season on the rival programme.
"Britney's bottom line was no less than USD 13 million", a source said. However it is also believed she initially asked for as much as USD 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 Lopez 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 Spears 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.
"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," a source said.