I'm not sure how common this is anymore, with the way prom has gotten bigger since I was in HS -- but when I was in HS, almost all of our "prom" dresses were really marketed as bridesmaids gowns and sold at bridal stores.
When I got married, we ordered the bridesmaids gowns online through PearlsPlace.com -- they had a great price! If your DD finds a BM gown online she likes, she might be able to contact Pearls Place for an online price.
I'll also ditto Nordstroms online, I've ordered formal dresses from them before for other events. And BananaRepublic online as well.