The Backsteet Boys are winning big in Sin City ... because we found out how much they are raking in with their Las Vegas residency ... and it's a huge figure.

Sources with direct knowledge tell TMZ … the band is grossing $4 million a night for their ongoing residency at the Sphere.

Here's how they're pulling in that much on the daily ... our sources say there's not much overhead or significant expenses involved -- they paid a one time fee of about $7 to $8 million for all the graphics they use in their shows -- and they've more than recouped what they spent on creative.

The Sphere has about 17,000 seats and the shows are selling out ... but the real key here is all the VIP tickets, platinum tickets and travel packages that go into the pot and contribute to their gross profits.

One Sphere insider tells us entertainers can make as much money at the Sphere as they can on a stadium tour ... and the fact the Backstreet Boys are only in Vegas and not traveling is helping their bottom line too.

Money talks ... and right now it's singing Backstreet Boys tunes.