My son is going to another school dance tonight. They hold 4 per year, and they charge the students a $5 admission. They also have concessions available to purchase, so figure another $5 for that. The dance is held for only 2 hours. There are about 1,000 students, most of which attend. So the school takes in about $10,000 per dance. The teachers voluntarily chaperone the event, so the only cost to the school is the electricity in the gym, and maybe some overtime for the janitors.
That's $40,000 per year, not a lot, but a heck of a lot more than any bake sale or fund raiser my school did when I was a kid. (yes, even in inflation adjusted dollars) 
