It looked like a Jackson 5 reunion ... when a Michael Jackson impersonator got together with 4 Jackson brothers Saturday at their concert in Vegas -- but it cost him a pretty penny.
Carlo Riley -- an MJ tribute artist -- tells TMZ he finally got to meet Jermaine, Tito, Marlon and Jackie at their Planet Hollywood show ... but he had to pay for the privilege.
Riley says he flew to Vegas from Denver and dropped $400 dollars for a VIP package that included front row seats, an autographed program and a backstage meet-and-greet with the Jacksons.
Carlo tells us the backstage visit was quick ... he shook hands with the brothers who told him they were impressed by how much he looks like their brother. And during the show, he says the Jacksons pointed to him in the front row ... where he was dancing and taking photos with other fans.
Pretty cool ... but it still can't compare to the time he met Michael himself in Japan in 2007.