Spears' representatives have been negotiating with Fox for weeks, a report said Wednesday.
"Both camps were back and forth on the money but now they have finally come to agreement on the fee," said one insider with intimate knowledge of the pending deal. "That was the big sticking point and now, thankfully, it's been sorted. All they need to do now is work out the smaller points. The fact the money is now set, means that there is nothing stopping this from becoming a reality."
Spears' fiance, Jason Trawick, who is also petitioning to become co-conservator of her estate with her father, will become a producer on the show, a second source said.
"Jason is expected to be there every day Britney is on set. He is her rock and will help guide her through the process. He is the person she trusts the most," the second source told E! News.
The deal would make Spears the highest paid judge on reality television, eclipsing singer-actress Jennifer Lopez's $12 million deal with "American Idol."
Cowell, an executive producer on the televised talent show, fired former judges Paul Abdul and Nicole Scherzinger in January after the ratings for the first season of the U.S. edition of the international hit weren't what he expected.