Have the bride buy your dress and use that as your gift or spend $100 on a dress that you will wear once and have a more personal gift. This is stemmed from a discussion I am having on facebook with some friends. Personally, I think if you are okay being in the wedding party, you are okay spending $100 bucks (or whatever you can afford) on a dress. The bridal party gift is a gift for saying thank you for spending all this extra time and money on me.
My Grandparents on their wedding day.
Repeat this to your self: My Wedding Party is made of my family and friends and I should treat them as such.