Be certain that the expense estimates are neither too lean nor too high. If you underestimate costs, you may not able to operate within the budget. If this occurs, you will have to go back to funders already supporting the project and ask for additional assistance, seek new donors, or underwrite part of the cost of general operating funds. None of these alternatives is attractive. Likewise, consistently overestimating costs can lead to other problems. The donor awards a grant with the expectation that all of the funds will support the project, and most will instruct you to return any funds remaining once the project has been completed. If you have a lot of money left over, it will reflect badly on your budgeting ability, and might affect the funder's receptiveness toward any future budgets you might present.
Remember to consider inflation or increases in costs, such as rent increases or annual salary increases. Certainly, funders don't expect perfection, and a certain degree of variation is acceptable, but if there's no way of knowing how much something is going to cost, it should not be included in your budget.
