In order to start the answer, I have more questions: What all do you (and kids) expect prom to include?
Some will say it is not prom without ticket, clothes, limo and dinner. Our local schools have prom tickets priced $25 (Senior class) and $75 (Jr class) each.
Last year, I spent $200 on a formal for DD to wear to the JROTC ball and she had planned to wear the same dress to prom (different school and different dates). The $200 dress got torn so we had to go to
Amazon to find another dress. $50 plus a bit of alterations.
That's another cost difference between boys and girls. Alterations! Tux rentals usually come with minor alteration, but girls have to pay extra.
2013: Both DSs skipped the tuxedo rental. One opted for his 3 piece suit (jacket, vest, slacks) with tie and pocket hanky. The other went with black slacks, tuxedo shirt with bow tie, and black suit jacket. Since they already owned the suit jackets...our out of pocket cost was $75 for both boys.
I cooked dinner for the boys and their dates (ok, the dates' families also), but the prom kids had the special dining area.
When DD went last year, I offered to cook. The date and his family said they were going to feed her. They did fast food. Not my choice and I don't think

thought that was the plan, but she was graceful.
Oh don't forget the cost of flowers, if you are planning corsages. DD was given artificial flowers for all of the dances she attended. She has kept all of them over the year.
Photographers are usually at the dance. It is nice to have that reminder, but if you can't swing the package price ($45 for starters), a camera in the pocket works great.
I think being fair is a good thing, but I can tell you there is no way my children have received the exact amount of money when it came to these excursions and dances. By starting the conversation with your teens now, you are including them in the decisions.
If I had to do it again: Boys would each receive clothes, flower allowance and $60 for dinner and the price of the ticket for himself and the date. Girl would receive the clothing, $60 for dinner and her ticket price. (If her date buys the ticket, she can use the money for photographer) I am traditional, but I expect my daughter to have the money in pocket to cover what she needs.