So, shooting in actual locations costs a ton, not just for the rental and insurance, but also because you're moving a ton of people and gear around. It's hard to explain but small increases in amount of gear and people can add orders of magnitude to the time it takes to get moved in and setup. Even on smaller productions a lighting setup for a large location can take several hours to create. You can spend half a day just getting into a location and getting it lit. On a big production, especially one where you don't have the benefit of being able to damage ANYTHING, it can take several days. Then, when you're done shooting, you spend half a day breaking down and moving to the next place.

Here's how to double your budget quickly:

Camera dept: "We want to shoot on this awesome camera, it'll only cost us $###,### more over course of this 10 week long production" Producer: "Okay let me talk to G&E (Grip and Electric dept.)" G&E: "ISO what now? That'll mean a lot more lights and generators, stepping up from one truck to two means another $x million in gear and people. Does the production dept. know about this?" Producer: "Hey production, we're shooting on x camera with two generator trucks and two lighting trucks" Production dept.: "Well, the extra crew and permits for that will cost us another $###,###.. Also, insurance needs to be adjusted on the order of $##,###.. Also, you checked with art dept. that all the costumes we made will be good with that camera right?" Producer: "Heeeeeey Art dept, can we get your costumes back for a camera test?" camera test goes terribly Art: "You need them all remade in how little time now?????" Sound dept: eats donuts