my bm dresses were pretty cheap - $50 @ white house black market. one bm already had hers, my sis bought her own, and i paid for the last one.
i think it depends on what else you're doing. the bm that already had the dress was given $500 from us for travel expenses. my sister's expenses were paid for mostly by my parents, so i wasn't too worried about her, and the bm whose dress i paid for had her flight paid for by her in-laws...since no one is close i don't have to worry about anyone talking about $$ while we're there, but i'd take each person's monetary abilities into account.
(could you follow that? it's confusing to reread!)