There is no single number, but there is a shape. Once you know the shape of an event budget, you can price your tickets from evidence instead of hope.
The lines everyone remembers
Venue hire is usually the largest single item and the easiest to quote. Sound and lighting is the second. Catering, if you are feeding people, scales directly with headcount and is the line most likely to grow after you have already sold tickets.
The lines that break budgets
Power is the one first-time organizers miss. If the venue does not include a generator with enough headroom for your sound rig, you are hiring one, plus diesel for the full run including sound check.
Security is not optional at scale, and the cost is per head per hour. So is cleaning, before and after. So is the cost of a stage manager if your programme has more than three moving parts.
Payment processing
Card and transfer fees come off every ticket. Decide early whether you absorb them or pass them to the buyer, and say so on the checkout page. Surprising people at the last screen is a reliable way to lose the sale.
Build the budget backwards
Start from the number of tickets you are confident of selling, not the venue capacity. Multiply by your ticket price. That is your ceiling. Now allocate: if fixed costs eat more than sixty percent of a conservative sales estimate, the event is too fragile and either the price or the scale has to change.
Hold a contingency you do not touch
Ten to fifteen percent, ring-fenced. Something will need it, and the alternative is paying for it out of the money you owe a vendor. The organizers who survive their first bad night are the ones who had a buffer.