Britney Spears Shacks Up with Sam Asghari in $15k-A-Night Hotel Suite in Las Vegas

Britney Spears spared no expense on a weekend getaway to Sin City with her fiancé Sam Asghari ... shacking up in a lavish hotel suite with its own backyard and pool!!!

Britney went all out for a two-night stay at Resorts World Las Vegas ... where she stayed in one of the casino's over-the-top "Palace" suites ... which go for $15,000k per night.

Here's what $15k gets you ... 3 bedrooms, plus an entertainment room, game room and a common area opening up to a private yard complete with a pool, grilling area, waterfall and dining area.

Britney made the trip with Sam and her agent, Cade Hudson, who snapped a bunch of selfies with Brit in different masquerade ball masks.

When she wasn't in her palace, we're told Britney and Sam got a sneak preview of Crossroads Kitchen Las Vegas before it formally opens over Memorial Day Weekend, and she also went shopping at some of the stores on the property.

It sounds like Britney's taking a liking to the property ... we're told she previously stayed there in March, so she came back for seconds.

The feeling seems mutual ... Resorts World Las Vegas had a special welcome message for Britney and Sam ... plastering the phrase "Resorts World Loves Britney" on their 100,000 square foot LED screen, which Britney and Sam could see from their suite.