C.B.
I think if she couldn't afford it, she wouldn't do it. It's just like any other party or gathering - the host foots the bill. It might be nice of you told her you would like to chip in, but then she might think you want to be a co-host of her gathering.
I think I would just relax and enjoy myself. Again, it's not like this is mandatory so she probably wouldn't be doing it if she couldn't afford to.