When working on an event, you should make sure to plan and include every expense carefully to avoid any costly surprise. The easiest way is to create a spreadsheet where you can list everything you need under categories such as venue, food, decoration, entertainment.
To give you an idea, here are some of the most common expenses:
Venue: location rental, equipment rental, tables and chairs, Audio/Video equipment, Audio/Video staff, security, barricades.
Food and Beverages: food, beverage, catering staff, bartenders.
Decoration: lighting, linens, decorative items, flowers, furniture, red carpet, step and repeat.
Entertainment: DJ, DJ equipment, band, performers.
Of course, this is a non-exhaustive checklist but it is a good start to create a rough estimate for your event budget requirements.