Project Management, during the planning phase, estimates the budget required to complete the project successfully and on time. The budget rests on a few figures: Cost from resources and cost from expenses. By carefully managing resources, the budget monster can be tamed. Resources are the single biggest cost for projects. Project managers know how much they've allocated to their resources, the amount of time that's been scheduled, how much that time costs, and what expenses are expected. They can also nip overspending in the bud before it starts and level resources if overtime is becoming a problem.
Useful budgets are created by a team, not by a single person. This way, the project manager can be sure her project plan is as accurate as possible. Perhaps Tom sees an expense that Jane did not. The project budget should be comprehensive.
Another thing to note about the project budget is that it is not to be confused with project cost. For a very nice discussion of the distinction between project budgets and project costs, you may wish to read "Creating your project budget: Where to Begin?" by Shelley Doll.
Finally, good project managers will aim to come in underbudget for their project expenditures, not at budget or overbudget. This ensures that they will be remembered for being efficient in managing time, costs, and resources. It will make it more likely that a client or customer will come to them again, with another project.
