Inexpensive prom dresses?

I'm going to prom this year for the first time and I'm only a sophmore. I want to find a really pretty dress but I want it to be inexpensive because my mom said she would pay for it if it was fourty or fifty dollars, but anything over that I have to pay for. Any ideas on where I could find a good deal on a pretty dress? I live in Minnesota, around the plymouth area.