I've worked out a deal with my close friends that we won't do any of these types of parties, because if we all feel like we have to buy something. So when I do get an invitation, then I know it's not a close friend, so I "have something going on already that night" which is not a lie, spending time with my family is my "something that's going on." If she is a REAL close friend, then talk to her and just tell her that you feel really uncomfortable at all of thost kinds of parties... What can she say about your feelings? If she doesn't care, or thinks you are not being supportive of her, then she isn't a real close friend, but one that you can sluff off to the category of "acquaintence."
Isn't it funny how we are so quick to try to be supportive and not make other people mad when we don't want to be somewhere in the first place, but then that puts the most important people in our lives second?