Wow, a real movie question.
First off, quantum physics is more understandable than studio accounting. Hollywood has a long history of cost inflating business practices, hidden money schemes, and other business practices that were pretty much eliminated from other industries back in the ninetieth century. The short answer is that the Hollywood studio system is rigged to stuff tons of money into lots of pockets and not into the movie itself.
For Signs, the production company leased a chunk of land from a college in Pennsylvania. They did this so they could control the corn fields which were vital for the movie. Since movies shoot on a schedule, they had to make sure the corn was at a certain height at a certain time. They also built the exterior of the farm house. The house gets pretty well trashed during the course of the film and there was a lot of night shooting. Having your own house to do with as you pleased meant you could shoot when you wanted without disturbing the neighbors, or worrying about punching holes in some historic structure.
And the house set itself is amazingly cheap. All you have to do is to make it look good on camera. Theres no interior. The thing is made from plywood. In fact, I think only three sides were even completed. Sets are generally pretty cheap, much cheaper than buying a real farm house.
The cost comes from the people. Mel and M. Night dont come cheap, but theyre a small part of it. There are the producers, the studio executives, the drivers, the costumers, the laborers all those hundreds of people whose names roll by during that intolerable twenty minute closing credits for any movie these days. Each one of those has their own union with their own sets of rules and wages. As an example, to get a lamp on a set and switched on takes members from six different guilds. And since this was location shoot, all these people had to be provided with hotels and per diem as well.
Then youve got the equipment, all the lights and cables and trailers and craft services table. Not expensive in their own right, but with all the studio mark-ups and other requirements the cost skyrockets. None of these costs make the movie any better; it just makes a lot of people wealthier.
The reason something like My Big Fat Greek Wedding can be made for only $5 million is that it avoids all the studio money traps. People do full jobs, not the guild mandated fifteen minutes of work (at triple overtime). You rent a car from the local
Avis dealership, not buy it outright from the studio transport pool. And the actors share a make-up table, not lounge in a doublewide trailer.
I should also comment that Signs had some issues during its production that added a significant amount to the cost of the film. Disney management forced some changes onto the film which required sets to be reconstructed and scenes reshot, the special effects were scrapped and redone, and the film was being re-edited to the very last minute. All of these added tremendous cost to the film (and left a lot of bad feelings all the way around in addition to weakening the film itself). Thats the other price of working with a studio people in suits think they know how to make things better.
Right now the conventional wisdom is that the average studio movie costs $85 million to make. By this time next year that number will certainly top $100 million. Already $150 million dollars movies are common place. Based on these figures, Signs is a low budget studio film.
P.S. The DVD contains an excellent scene which was deleted because of studio issues. It takes place upstairs underneath the attic just as the aliens are invading the house. There has never been a scarier closing door.