My suggestion was to check the clearance sections of the department stores she was shopping at because, chances are those dresses are from previous seasons and will be highly discounted. Plus, no one will have the same exact dress as you and you'll be unique! If you shopped the in style trend dresses, chances are other people are as well.
My suggestion worked out for her because she was able to find a dress for only $20, originally $100+ something! She was the only one with the dress.
A inexpensive formal dress can be found by secondhand shopping, if you have time to look. This page is about finding a cheap prom dress.