Get a black cocktail dress for about $50 at JCPenney. OR...if you have a Burlington Coat Factory near you (OMG. I could live in that store!!!)
Also try:
Craigslist
Bridal stores (oftentimes have prom dresses, bridesmaid dresses, etc. for sale)
Goodwill
Consignment stores
Also try:
Craigslist
Bridal stores (oftentimes have prom dresses, bridesmaid dresses, etc. for sale)
Goodwill
Consignment stores