It doesn’t cost that much... The people that owned my house prior to me converted the garage... I turned it into a theater... Projector cost 1500... Screen cost 250 and I already had a DTS stereo with Klipsch 5.1 speaker set-up... threw in a couple recliners and I never go to the movies anymore...
I know you can get a pretty decent set up for far less. A 4k player, nice screen, and a surround system and a comfortable chair can all be had for under $5 grand, but a lot of people like the movie experience and pay a lot to have it.