Collect data, talk with other show managers, ask your general contractor lots of questions, ask your convention center and hotel contacts for help. The more you learn, the more accurate the budget will be. Pay particular attention to areas that can cause the most overruns, such as catering, printing and postage. Call your caterer. Don't forget to budget for gratuities and taxes, and check the exact gratuity and tax percentages in the facility.
Some states allow gratuities to be taxed on banquet checks.
That will make a major difference in your calculations, so find out what your facility does. Review past records to assess no-shows. If you have a new show, start building your history. Most importantly, review all costs up front, from carvers to bartenders to serving stations. Printing can be outrageously expensive and is easy to overrun.