Recessionist thought of the day: clothes for fancy events (like proms, dinner parties, awards shows, etc) should be purchased at ROSS DRESS FOR LESS, MARSHALLS, OR THRIFT STORES. The "thrift stores" angle is especially true if you live near a high-end area.
Why? Frankly, the retail version of an outfit you're going to wear one time (maybe twice) is probably a worse deal than buying a brand-new car (percentage wise) in terms of depreciation. Granted, it might take some convincing to get your teenager to check out a prom dress at a thrift store, but here's a thought-- before settling on a final outfit, check out at least one full-price dress (like at Macy's), and one thrift store. No price-related decision is ever final until more than one option has been explored!