I'm glad you saw the light before you ended up going. She was playing you for a fool. As I read the above and thought about how this unfolded, I couldn't help but again think about how transactional and shallow it sounds to think of basing whether you spend money and time taking a girl on a trip on whether she is going to have sex with you, but what just happened to you is the proof as to why men have gotten to a place where we have no choice but do this in order to protect ourselves because so many women are dishonest, manipulative, narcissistic users. A man needs a way to figure out if a woman is truly interested in him or not and unfortunately (or fortunately, depending on how you look at it) sex is one of those ways. Like I said before, a woman can still use a man while she is having sex with him but at that point, it's generally only if he is doing a lot more than taking her on an inexpensive trip. Sex is one of the only ways to vet women's true interest in you. 98% of women would probably try to shame you for thinking like this but those same women created the problem by using men.
I'm glad you didn't give her a reason as to why you didn't go but if she asks again, you should tell her you're taking another girl you've been seeing instead.