Many Hollywood filmmakers read a script and look for the most dramatic or climactic moment in the proposed film. Regardless of the costs or technical challenges of filming such an impressive scene, producers and directors will do whatever it takes to get that shot completed. Because of its box office importance and expensive set-up, this climactic scene is often referred to as a money shot. A money shot is not necessarily the final scene of a movie, however. Sometimes the first appearance of a movie monster or the first glimpse of a Russian submarine or doomed luxury liner would be considered a money shot, since it's the one scene audiences will pay money to see.
The origin of the term money shot has its roots in a different type of film industry, however. In the adult entertainment industry, male performers are routinely asked to provide cinematic proof of the scene's authenticity. Some actors even refrain from sexual activity for several days in order to fulfill this particular obligation. Suffice it to say, the "money shot" of a standard adult film leaves little doubt in the audience's mind that a real sexual act has taken place.
Legitimate movie production companies have successfully co-opted this unfortunate piece of jargon to represent a much different type of shot. A money shot can be one of the most expensive scenes to film, or it could be a critical or pivotal scene which must satisfy the audience's expectations. In the Harry Potter movie series, for example, a dramatic confrontation between the heroic Harry Potter and his nemesis Lord Voldemort would be considered a money shot, since fans of the book series would be disappointed if that scene lacked enough action or strong visual effects. The money shot of any movie would be the one audiences discuss on their way out of the movie theater, and the one they would recommend to future audiences.
It would not be unusual for modern directors and producers to include several money shots in the course of a two or three hour film. Sometimes the sheer immensity and expense of a money shot requires severe budget cuts in other departments. This is why many action/adventure movies or epic science fiction films only have a few A-list actors in their casts. The money which would have ordinarily gone into a recognizable actor's salary is instead funneled into the cost of the film's crucial money shot. Shooting with a relatively unknown cast or limiting expensive location shoots can usually provide enough money for filmmakers to set up and execute an elaborate "money shot" or two. Sometimes a director will only have one opportunity to get the shot right, so a lot of time and money is needed to make sure all the prep work is done correctly and the shot goes off without a hitch.