A bridesmaid dress is really only going to be worn once. For this reason, some believe that forking out a lot of money on a dress is just not worth it. Some brides will pay for their bridesmaids dresses and some will expect their bridesmaids to purchase their own. If you are a bride that is opting for the latter I believe the best thing to do is to sit down with all of your bridesmaids and have a chat about what would be reasonable for everyone to spend (and afford).
If you want your maids to wear a dress that is not within their budget, then you may want to consider paying for some of the cost, or at least pay for their accessories e.g. shoes, jewlery etc.
Remember that you have chosen them to take part in your wedding so be considerate and avoid being a bride-zilla!!