Mariah Carey & Elton John Rake In $4.2 Million at Russian Wedding

Mariah Carey and Elton John got hired for a wedding gig over the weekend ... and it wasn't cheap.

The singers raked in $4.2 million after a Russian billionaire, Valery Kogan, footed the bill for his granddaughter's wedding over the weekend in London. The astonishing 9 hour reception at The Landmark hotel featured Mimi dedicating "We Belong Together" to the new couple.

The bride -- 19 year old Irene Kogan -- and her hubby requested Mariah's appearance. She was flown in from the U.S.

Sir Elton John kicked things off with an impressive 12 song set. And listen to Mariah ... she sounds fantastic.