I agree with "FMS", when you accept the BM invite, you accept the expense. As a bride, I wouldn't be upset if you didn't get me anything, but as a guest or BM I would feel weird showing up with nothing. I always give at least $50 for me and each guest ($100-$200). If you don't want to spend too much, I would give her an actual gift instead of money, that way it's more thoughtful.