It's a buyers market right now with all the theatre closings around the country.
I was to a multi-plex auction last spring where you could have equipped your booth per screen with digital stereo sound, platters, projectors, lamps... everything for about $5000. Most of the equipment was about ten years old except the sound equipment which was only about three years old. I think for the most part though when using good used equipment the cost should fall somewhere between $5000 and $10,000 per screen.
If your not interested in stereo, and willing to use old equipment, between 25 to 50 years old, but still good, then you can get away with about $2,500 to $3,500. Some of that equipment made 50 years ago is still better then a lot made today. I just bought 25 Simplex E-7 heads in good condition... boy are those things workhorses. I only needed a couple as backup or for parts, but had to buy the lot. Anyone interested in any E-7s?