For the purposes of positive thinking, let’s make some basic assumptions. The project of all your attentions and passions has found financing, but it is conditional on your attracting a director and cast that the financier (not a major studio) will approve. These elements must fit within the budget you submitted with your script, which you believe is sufficient for you to make the movie you are promising to make.
Again, if this is not the case, then you are starting out in a bad position. It is unwise to begin a film if you are not confident in your ability to produce the movie properly with the budget you are being given.
Budgets do rise, and financiers often find themselves anteing up more than they planned, but it’s not a pleasant process. In fact, it’s distinctly unpleasant, and should be avoided whenever possible, especially by speaking up early in the process.