Since you're still unsure, I would just tell them what you've told us. That you have yet to decide if you'll actually go due to cost and other trips you've already planned.

Better that she'll know you might not actually go, and im sure she'd be pleased if you decide to. At least she won't be expecting your arrival, it would lessen the dissapointment if she were to feel that way, as opposed to telling her you're going when you're still undecided and can possibly change your mind.