I asked a Alamo guy about this...
You pay whatever for the theatre and then its $100 per person min for food.
So maybe $600 at the low end (4 people at $200 for the room plus $100 each for food) for a few people to watch a movie....
Frankly, just go to the Alamo during off hours and it's close to being a private theatre anyways... get out of there for 5% of the private option.