First and foremost if they are true friends then they will understand why you can't and don't want to go. That is a lot of money to ask someone to spend for their birthday, plus the fact that you don't want be out in the cold with your fibro and arthritis. I would call the friend with the b-day and tell her why you can't go and set up a day to take her to lunch or dinner for her b-day. That way you are still acknowledging her special day but don't have to deal with the cost and pain. Again if she is a true friend she will understand, and if she isn't then that is her loss.