Out Of Town Party Big $$ How Much Is Too Much

My friend is in the wedding for her brother and soon to be sister-in-law. The wedding is in New England and that is where everyone is located but the Bachelorette Party is going to be in Arizona because family have a vacation home there for free. Everyone is required to pay for plane, two nights out etc (on top of wedding apparel, beauty and gift etc) The Maid of Honor has now asked everyone to come up with more money for a limo (70-100 a head) among other things. They are each paying 500+ a person already. My friend is a college student living with City rent on a fixed income. She cant afford a lot because of the things for this wedding she still has to buy, dress, shoes gift etc. Now she is uncomfortable b/c she doesn`t really know any of these people, but she wants to go to show her future sister in law that she likes her. How much is too much, how does tell these people she cant afford it without coming off cheap? Has anyone been in this situation before?