Some girls do spend this kind of money, but the average girl probably spends $300 or so. Yes, that's still an outrageous amount for one evening, but it's a far cry from $1000 and it's a highlight of their high school experience.
A few girls really do it cheaply -- almost free except for tickets. A borrowed dress, shoes they already own, a single rose or a carnation corsage, skipping the limo. And some people even go out to dinner at places like Taco Bell. I knew of two moms last year who went to Taco Bell ahead of time and put out tablecloths and flowers, ordered ahead of time, and then their kids' group arrived and "dined" on tacos! Apparently it was the hit of the evening, and everyone wished they'd been in on the taco meal. Thing is, in the long run, no one can tell the difference between the cheapskates and the $1000 girls.
Senior year my own daughter went with a guy-friend. She had a $50 dress from a clearance table (looked GREAT on her and is something she might actually wear again, say, on a cruise or to a college dance) and shoes and jewelery that she already owned. They went dutch for tickets and dinner, and she had a great time.
What makes me sad, however, are the girls who THINK they can't do it because they can't afford the big, big money -- so they miss an experience that they wanted because they don't realize it can be had for next to nothing. Kids talk and talk about it, and some kids get the idea that if they can't play in that league, they're just OUT.