I am a bit on the fence on this.
On one hand, I feel it is important for children to learn that they need to live within their family means and cope with disappointment gracefully.
On the other hand, I don't see why some children should be 'rewarded' with trips, just because their parents are willing to pay for it. Comparing it with being selected for the choir or cricket team is disingenuous, because the children have to work hard and make the effort to get entry into competitive sports.
I think, like a poster suggested earlier, that the whole year should organises a fund raiding activity. Then based on the funds collected, the children should be divided into groups to come up with a 'business plan' on where to go, the cost and benefit' and one of these selected as the winner for the class.
This way, not only would all the students get an outing, but they would learn valuable skills of teamwork, budgeting, persuasion, and most importantly, learning thatbthey are part of a large whole, not individuals out to grab what they can for themselves.