If he really wants to go,

you can ask him to pay partially. What % will depends on the agreement between you and him. He can sell things in school to raise his part of money. It's a good experience. Don't set the goal of "found raising" too high, and it enjoyable and reachable.

My son did that for an very expensive summer camp. It's not that i can't offord, it's about gaining experience and learning the value of earning money for good cause.