When I was in weddings, I had always assumed that I was going to pay for all the items that I would need to be a BM. For our wedding in September, I took all the girls to a national bridal place and told them that they could pick out their own dress they just had to be the blue that I picked out and floor length. This was a way that they could pick the dress that fit them and their budget the best. Another suggestion would be go to the mall after prom. There are dresses that are marked down to reasonable prices (as long as you can find one that you like).