The Budget of a convention allocates expected income into categories of expenditure and makes it possible to estimate the convention's "bottom line".
The ConChair and Treasurer should put forth a budget with input from the rest of the concom, and have it approved by the Executive Committee or Board of Directors. Heads of departments should be given the budget and are expected to follow it. Any expenses not initially outlined in the budget should be approved by the ConChair before they are spent.
Setting a Budget
When in doubt, start from last year's budget. If you are a new convention, see if an existing convention that is of a similar size and shape will share knowledge with you.
Make sure your budget has some padding, which is to say you should overestimate your expected expenses at the beginning. Then as actuals come in throughout the year you can adjust it to get more exact. .
An excellent tracking tool for budgeting is to plan in advance When you expect to spend the funds, in addition to How much you expect to spend (a Burn Rate estimation). This will provide the opportunity to work towards a monthly / weekly / daily budget as the convention approaches, and to spot deviations from the plan when they occur. Estimates of the seriousness of the deviation can then be assessed and corrected for very early, before they cause funds shortfalls near the end of the convention, when it is too late to do anything.
Try not to overestimate your expected income. Whenever possible let numbers from previous years be your guide. Note that exceptionally fabulous guests will probably not increase your membership as much as you are tempted to think.