What's worse than underage celebrities in nightclubs? Being an underage celebrity in a nightclub that doesn't comp you.

Mop-topped Jesse McCartney and girlfriend Katie Cassidy, both 19, hit L.A.'s Privilege with some pals Tuesday night. According to witnesses, the crew lived large on Cristal and Grey Goose.

By the time they were done, their teenage tab came to a very grownup $2,500.

"When [McCartney] saw how much it was, his eyes got wide and he said, ‘Holy s-!'" a spy tells Star magazine.

But the final insult was when Nicole Richie's former handbag, Brody Jenner, had the last word.

"Welcome to the adult world buddy," the fellow club-goer told McCartney. "That's what it costs to have fun with the big boys!"

McCartney's rep told us: "He was indeed at the club, but the rest of the story, as it was presented, is untrue."