Typically bridesmaids pay for the dresses, so that's something else to keep in mind. I had a friend who chose relatively pricey dresses, so she generously pitched in some of the cost. The wedding party is also expected to pay for their own hair and makeup, so it's fair to give them the option of doing it themselves. As for other necessities on the day of the wedding, like transportation and lunch, it's a nice gesture to treat the wedding party if you can.
Source: Flickr User imothyhorrigan