Let that person know that you appreciate being included and want to be able to go on the trip with them, but you do have limited resources. Ask what the expectation would be for expenses during the trip. This may feel awkward, but it's way better to learn now than to be away and unable to participate or contribute fully.
It's never too early to learn that while it's good to think compassionately about what's happening to others, ultimately they own their behaviour and you own yours.
Understanding your friendships from a perspective of financial inequality can help to realize that wealth often comes with hidden costs, including emotional stress and personal sacrifice.