If she is not concerned about your situation, maybe she is not that good a friend? Or maybe she is just wrapped up in the wedding, and really doesn't know what you are going through. I would be honest and tell her your situation. If she is a real friend, she will understand.
Why do you have to give her a $100 gift? You have to put your daughter first, and if that places a severe financial burden on you, then don't do it. Again, your friend should understand.
Tell her you would like to celebrate her wedding, and come up with a plan to do it with her in a less expensive way. I liked the idea of inviting her over to dinner.
If you would want to go to the party, if you had the money, then tell her you would like to attend, but cannot afford a gift or to spend any money while there, and maybe she will say, oh just come and have a good time, and it won't cost you a thing, I will take care of it.