You should look at consignment stores. You can find major dress designers at a fraction of the cost. Many people have only worn the dress once and then have no use for it, so they decide to consign it. That way you get a beautiful designer dress very inexpensively.
I would check out Forever 21. It is a great store that has good quality clothing for rather cheap prices. There is one in New York City and other various locations. Debbs is also another choice that has cheap dresses!
A great place to look for cheap formal dresses are consignment shops. You will typically find previously worn dresses at these type stores, but most are in excellent condition and will cost a fraction of the price of a new dress. Another great way to find a new cheap dress is through ebay.