A source close to Demi told The Mail on Sunday newspaper: "With a pre-nuptial agreement, Ashton should be sitting pretty.
"These agreements are designed to protect one's earnings during the marriage as well as one's pre-nuptial assets.
"But Demi is furious, raging in fact. You only have to look at every photo you see of her to see the toll Ashton's cheating has taken on her.
"She wants to punish him financially because of the way he has broken her heart. She is talking about going after a share of what he has earned since their marriage on the grounds that she contributed to much of what he has achieved.
"She is going to give him a bitter fight and, whether or not she wins, it will be an expensive lesson for him."
Demi and Ashton's marriage was rocked when 23-year-old Sara Leal claimed the actor had sex with her at a San Diego hotel room in September.
The couple are thought to have tried therapy sessions in an attempt to save their marriage, but Demi decided she couldn't forgive Ashton.
The source added: "She was a wreck. She believes strongly in marriage and she really didn't want to spend her middle years alone. But in the end, she just couldn't see any way of making things work."
- BANG! Showbiz