When I was that age, my parents were on a limited budget due to paying for my private school tuition. I had a job and was expected to pay for car expenses (insurance, gas, etc), any clothing beyond the basics (my parents paid for underwear, socks, jeans, t-shirts, school uniforms, but I paid for any fancier stuff that I wanted). They would give me pocket money of about $100/month (this was 20 years ago) so I could go to a movie, do something fun with friends, etc, but anything past that, I had to pay for myself. They paid for college application expenses, though they did make me pare it down to 3 schools before I applied.
Basically, I think our job as parents is to give our kids everything they NEED (education, basic clothing, food, love and guidance), and very little of what they simply want (a new car, prom, spring break trips, entertainment and hobbies). I think it's great for kids to learn that those "extras" do cost money, and to pay for at least some of these items out of their own money so they have a sense of how many hours they have to work to pay for those extras.
I would say with regard to prom, that dress is more than enough for you to pay for. Can she wax her own eyebrows and lip, do her own makeup and hair? I mean, when you grow up and need to go to a fancy event, you don't go get all that stuff done. Is it really integral to the experience? Somehow I doubt it. Does she have shoes in her closet that she can re-purpose for prom? Borrow a pair of yours? It doesn't all have to be crazy expensive in order to look great! Just my thoughts. :)