Remember, $/person is only HALF THE EQUATION!
Price = $/person times # of persons!
Add up ALL of your costs, including what you want to earn. That equals the MINIMUM $$ you must charge to avoid losing money.
Now divide that by the number of people expected to get the $/person WITH A MINIMUM NUMBER OF PEOPLE EQUAL TO THE NUMBER YOU DIVIDED BY!
Why? Say you calculate your costs and you would be satisfied with, um, $500 for the gig so you quote $20/person ($500/25 expected attendees). Five people fail to show up. Now you have $20/person times 20 people = $400, not $500, and you are in the hole $100!
If you MUST quote $/person, include the absolute minimum number of people to be charged for, i.e. 25 people @ $20/person.