Expensive prom dresses...
Unusual, and usually expensive, modes of transport to get there...
Expensive venues...
Expensive food..
....and extremely expensive soft drinks [bearing in mind they are too young for alcohol]
The last one held by my daughters school was charging £25 a head for mediocre, luke warm buffet and a disco at a local hotel. Orange juice was £1.50 a glass....
The kids were arriving in stretch limo's etc....the lads had mostly hired dress suits, and the girls were all in ball dresses, with hair-do's and all the other expense that goes with it.
I wonder if the current economic times will encourage less such extravagance.
FYI, T, even the Greenwich Village Halloween parade is being toned down this year - there will be only a few floats, because the usual sponsors are cutting back. But I see that the stores are full of the usual candy, costumes, and related paraphernalia.
The difference from my childhood is that then the holiday was mainly for children, with mostly homemade costumes, and now it's been taken over by the adults, who of course spend lots more.