Is there an open bar? If so, go and drink your $s worth
Seriously, just like everyone else said, try to think of if you'd regret not being there in the future. If you won't, then tell her AHEAD of time. It will make you look bad to cancel last minute. I know we had some people do that at our wedding, and it cost us a lot of $ and I'm still mad at them. I know that what she did was hurtful, but don't give her any reason to be mad at you. Let her be the bitch in this situation.