If you must decrease a project’s budget, you could look at the grades of material resources for which you had budgeted. For example, did you plan to shoot a film in 35 mm when cheaper digital video would do? A lower-grade material is not necessarily a lower-quality material. As long as the grade of material is appropriate for its intended use, it might still be of high quality. As another example, fast food and gourmet are two grades of restaurant food, but you may find high-quality and low-quality examples of each.
You should also look at the costs of the human and equipment resources you have planned to use. Can you hire less experienced people for less money to carry out simpler tasks? Reducing project costs can lead to a poorer-quality deliverable, however. As a project manager, you must consider (or, more likely, communicate to the decision makers) the benefits versus the risks of reducing costs.