Most brides have their bridesmaids buy their own dresses. The bridesmaids help plan the showers, and help with all the wedding day festivities prior to and during the actual day. I had a good friend tell me that she was a bridesmaid in another wedding where she was in charge of all the wedding favors. She made over 300 sugar cookies for her friend's wedding which took over six hours! Now I would say that is above and beyond a bridesmaids duties. Being a bridesmaid takes time, money etc. and so as a bride respect your bridesmaids and get them something. They didn't have to be a bridesmaids you asked them and they accepted because they love you. Now depending how much the dress and shoes they had to buy you should make sure you give them a good gift for that. If you bought their outfits, shoes etc. then you can probably get by with getting them something smaller like some lotion, a giftcard, jewelry etc. The possibilities are endless! So don't worry about what to get them. Remember it's the thought that counts! It's nice to write them a nice letter of appreciation as well.
No matter what your budget it is PLEASE do them a favor and get them something, or they might just not like you anymore and never talk to you again ;) haha. Just kidding (kinda).