Bat mitzfahs and dresses?

Im going to my friend from camp's bat mitzvah who lives 5 hours away in january and well... im not jewish and none of my close friends are so i have no idea what to expect. We are supposed to dress elegantly so does anyone know where i should go to get a sort of expensive dress(Budget $450) because i saw some pictures of my other friend who lives across the country's bat mitzfah( i couldn't go) and know that everyone was dressed nicely. So please tell me what will happen and how to dress!!!!!!!