It would be good to be honest and acknowledge her feelings right off. Tell her you know she will be very disappointed, and you are too, but you just can't afford the trip this year. What about telling her what you said here, that you admire how she bakes, cooks, decorates and makes the holiday so special and that you're going to miss that.

She has to deal with her own thoughts and feelings about your decision.
Your decision is about what's best for you.