I’d note that a lot of Protestant youth groups also do these mission trips that high school or college students are expected to pay similar amounts for, if they want to go. So I’m glad to see a Catholic group offering it too, because there’s definitely interest.
The purpose of the trip is to have a bit of an “ethical vacation” while giving the participants some exposure to, and participation in, mission work. Obviously if they’re just going for a short time, they’re not going to be able to accomplish great things for the people in that span of time, but they get familiar with how people in a mission area live (very differently), it’s an eye-opener for many who go, and it increases the chance that they might financially support the missions in the future, or even go into mission work. The missions typically do receive some benefit from welcoming the tour groups. And it’s a lot better pick for a trip than going to some island and drinking and shopping for a week.
Sure, the mission would get more money if instead of going on the trip, everybody just donated 1000 dollars to the mission and stayed home, but realistically, students aren’t going to do that, and there’s a lot of value in having the hands-on experience.