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.

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.