I totally see why you'd be super frustrated with your friend. She goes on and on about how she wants to make it to your wedding no matter what, but yet can't make that effort to save by limiting spending. And I don't think you were asking her to change her lifestyle, she's the one that said she wanted to come!
However, I do agree with Erik too - we had the same attitude, if you can"t make it, I am sorry - we will miss you, and we'll send you pictures. We were not upset that anyone couldn't make it, whether or not they couldn't afford it, or just didn't want to spend the money to come to Mexico for our wedding. Their choice!
I do think this case isn't that simple though - this girl seems to have made it clear she wants to be there, and wants to spend her $$ to come - then backed out on that, and was given the free ride, but said she'd save up anyway. But then she isn't. I definitely would be frustrated too.
Could I talk in circles any more?