Shawn Mendes played his first Bat Mitzvah Saturday ... the crowd may have been a bit smaller than he's used to, but the price tag certainly wasn't.

Shawn serenaded 275 13-year-olds at an insane bash, honoring the daughter of J. Darius Bikoff -- the guy who created Vitaminwater and Smartwater.

We're told the party cost between $5-7 million. Grandmaster Flash DJ'ed and the Bikoffs also had 40-50 horse carriages waiting to escort the guests from the temple to the party, which was at the ultra-swanky Plaza Hotel.

It's not Bikoff's first rodeo. In 2013, J. Darius surprised his wife with a Justin Timberlake performance at their 10th wedding anniversary that put the millionaire back $1.3 million.