Hello! I am trying to really watch my budget too, and I just got a GORGEOUS GORGEOUS gown at David's Bridal. They have great sales. Mine was one of the more expensive sale items at 449 dollars, which is still pretty good for a dress. I suggest them if you are tight :-) Good luck!
You can also try looking at preownedweddingdress.com, oncewed.com, or recycledbride.com. These are websites where brides can sell their dresses (some are even new and have never been worn!) Otherwise yea, I would check David's Bridal. Best of luck!
They just opened a new Nordstrom Rack a month or two ago on Colorado and Florida. Some of the formal gowns there are beautiful as wedding gowns and at great prices (many under $100). Friends and family could get you gift cards to the store to help out and make things a bit more affordable. Otherwise, check Craigslist. You may just find your diamond in the rough! Good luck and congrats!