Can't be bothered to read all the millions of replies, so I'm sure I'm saying what has already been said. But my brother and his school friends did one of these trips, they are generally for their Gold Duke of Edinburgh award. Say the tip is £3k, about £1k will actually go to the charity, the rest is flights and food.
The trip is usually split in two parts, the charity part which usually involves building something, like a school classroom, or a water tower, then th second part will be some kind of trek involving living and working with locals, learning about the country/culture.
Part of gaining the award is that to be able to go on the trip they must do regular volunteering for x amount of time before the trip, a year maybe, plus another x amount of time doing a 'skill' such as a sport.
The whole idea of the award scheme is to get people to be better rounded, and have more than just qualifications, and a better understanding of the world. They are so many benefits for the charity, and the individual.
And the scheme is not limited to 'private school girls' the whole idea is that you HAVE to fundraise for the trip so that it is available for everyone, your parents cannot simply pay for it. The are tonnes of 'inner city kids' that do it and what they gain from it is invaluable.
Please get your facts right before bitching about it on here.