A multiplex is a concession stand which happens to show movies in order to lure you into the range of the smell of their popcorn.
It has nothing to do with movie theater monopolies. As it was explained to me by my manager, back when I worked in a movie theater in a small Midwestern chain, for every movie, the studios take some percentage cut of gross ticket sales, varying from movie to movie.
Star Wars: The Phantom Menace in 1999 was the first film for which the studio demanded 90% of gross ticket price — continuing a long-standing trend of raising the take which possibly began with the original first Star Wars movie. The other studios quickly followed suit and raised their take to 90%, especially for the big blockbusters — the textbook term is "oligopoly pricing" — and since then the percentage has inched ever closer to 100%.
I forget exactly what it was in the second Matrix movie or Lord of the Rings: Return of the King, both of which premiered while I was at the theater, but the number that sticks in my head is 94%. Obviously, the studios can't directly capture any revenue from the sale of popcorn — unlike the movie, it's not their product — so every time they raise their take, the theater compensates for lost revenue by raising the price of popcorn.
This trend hasn't reversed with 3D and IMAX and all the new technologies coming down the pike. The only reason they're attractive to the theaters is that the theater can charge $15 a ticket rather than $10. Even on a small percentage share, that's a 50% jump in revenue and covers the not insignificant cost of the projection equipment. 3D is also currently getting more butts in seats than 2D was, leading to somewhat more concessions sales — going to the movies is an outing and an event again — though that's tapering off as it becomes less and less of a novelty. The ticket prices aren't coming down, though.
Moral of the story: like razors or printers, theaters lose a ton of money to show you movies due to studio oligopoly pricing, and make it up on popcorn.
