TALK about high maintenance! Friends and family are pushing child star-turned-fashion mogul Mary-Kate Olsen to get a $500 million prenuptial agreement before she waltzes down the aisle with French-born investment banker Olivier Sarkozy.

Rumors flew that Mary-Kate and Olivier were already in a trial marriage when she was snapped sporting a band on her ring finger on Dec. 12.

And pint-size Mary-Kate, 27, was spotted shopping for a large engagement ring at the Neil Lane boutique in Los Angeles. She’s said to have her heart set on a huge sparkler.

“All of the rings she looked at are over five car­ats,” a source told a reporter.

Although the bling could set 44-year-old Sarkozy back a staggering $100,000, that’s nothing compared to the gigantic sums that would be protected by a prenup.

Celebrity divorce attorney Raoul Felder told The ENQUIRER: “She’s worth a bundle and he’s got money, too.

“So this would be like two corporations merg­ing – which is Sarkozy’s specialty.

“The prenuptial agreement would be worth, as the French might say, beaucoup bucks!”

Sarkozy – half-brother of former French president Nicholas Sarkozy – is worth at least $60 million and knows how to protect it. The father of two is already battle-tested after under­going a bitter divorce from his ex-wife, writer Charlotte Bernard.

Meanwhile, in November 2009, The ENQUIR­ER disclosed that Mary-Kate – who reportedly is eager to have a baby – was looking to protect the $900 million fortune she and her twin Ashley have amassed through their fashion and enter­tainment empire while pondering marriage to her then-boyfriend, artist Nate Lowman.

Predicted Felder: “A new agreement (between Mary-Kate and Olivier) could be the mom and pop of all prenuptials!”