Those sorts of trips are entirely optional. You're not "expected" to pay anything.
Our school (state comp in a leafy, affluent suburb) came up with the bright idea of a Netball trip to Dubai, less than a week for £2500. They couldn't get enough parents willing to pay it, so it was cancelled.
I can't really get myself worked up about how other parents choose to spend their money. If they're happy to pay £3k for a child to go off on a trip somewhere, that's their business.