In 1986 my dad paid about $60.00 for my prom dress. That was for the fabric and pattern because I made my own dress in Home Ec class.
The other girls? They might have paid around $200. or so. Hard to say. That dress I made would have been very expensive to buy.
Here was the pattern. I made View A just as you see it, in Red.
https://www.rubylane.com/item/135665-9019/Vogue-Bellville-Sassoon-Formal-Bridal-Dress
Wow!