The multi-million dollar reason Lady Gaga, Britney Spears and J Lo have gone all-in on Las Vegas …

Sin City residencies have been big business since Frank Sinatra and Elvis Presley held court in the 60s and 70s, and they're getting an encore as pop stars like Britney Spears, Jennifer Lopez and Mariah Carey have hit the Strip. Spears' four-year “Piece of Me” residency at the AXIS Theater at Planet …