It should be the amount of money it will take to produce the film, and not a penny more or less. There is nothing worse than a grant-funded film running out of money due to poor planning. It is also very hard if not impossible to obtain grant money if you have a track record of not completing projects. To apply for a grant or short film finance of one sort or another, you must first apply to the agency or authority and request an application form and guidelines. Read the guidelines very carefully.
Every funding agency has a different aim or purpose.
Don't bother sending your mini Die Hard project to an organization trying to encourage less violence in cinema. Try to see it from their point of view. Understand what it is they are trying to achieve by bankrolling short films, and tailor your application to address their requirements. The usual grant application will have several components. First and foremost, the script or treatment.