I hate proms! Just kidding but they really are money pits. My daughter who is now a freshman in college went to 3 of them in high school and I am glad she won't be having anymore proms. Her first prom (sophomore year) we found a dress at David's Bridal that was around $130 if I remember correctly. Her second prom (junior year) we spent a bit more and purchased the dress at a local bridal store. For her senior year prom, we bought one a year ahead of time that was on sale right after prom season was over. It was a $500 dress that we bought for a steal.
David's bridal seems to be the most reasonably priced for prom dresses around here. There's also a store here called Group USA that is okay on prices but I think they are higher than David's Bridal.
Good luck in your search, OP!