Tyler Cowen over at the Volokh Conspiracy gives some reasons why movies don't cost more on a Saturday night. But, in Los Angeles at least, they often do.
There are many theaters in Los Angeles that have variable ticket prices for different movies on different days at different times. Prices for different movies at the same time and day can be different, as can prices for the same movie on different days at different times.
Two examples are The Bridge in Westchester, and the ArcLight in Hollywood. At each there is a large scrolling display screen that gives the current ticket prices for each upcoming movie showing. It can be confusing, and most of the time you don't know how much a movie is going to cost you before you arrive (yeah yeah, unless you do research).
All of these theaters have student tickets, however, that provide quite substantial discounts and address some of the price effects that Tyler mentions. For example, a Friday night movie at The Bridge might be $15 normally, and $8 for students.