Credit: Jon Kopaloff/FilmMagic

How do you know when your movie's a box office disappointment?

When the opening weekend gross is about half the salary of just one of its stars.

Reese Witherspoon's How Do You Know did just that. Over the weekend, the romance debuted at number 8, raking in just $7.6 million in receipts, according to the Associated Press.

Costarring Owen Wilson, Paul Rudd and Jack Nicholson, the flick cost a reported $120 million (according to the Hollywood Reporter) to produce -- including a $15 million paycheck for Witherspoon, 34.

Sci-flick Tron: Legacy fared much better, landing at number one for the weekend with a $43.6 million gross.

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