I understand OPs point when they say "it shouldn't matter where it is"
Of course, a foreign ski trip will cost more than say a UK camping trip.
The point is, why has a state school located in an area that isnt known for its affluence, organised a trip to anywhere that costs that amount of money? Knowing full well that many children will have to miss out due to its cost.
If any trip, regardless of where it was or what it entailed, cost this at my DSs school I could guarantee AT LEAST 70% couldn't go at all. £900 wouldn't be feasible for a family holiday for a week for most of us, let alone one child for just 4 days. Many of the parents at our school are using food banks.
As a kid, I remember my school did something similar. 50% of the class couldn't afford it. I was in that 50% and I was laughed at and picked on for being one of the poor ones. And this was a topic-related geography trip that meant those that couldn't go also got a crap mark for the essay. It still angers me now.
I'd be very pissed off if DSs school did this, whether I could petsonally afford it or not, because it does exclude many others based on tgeir family's wealth, and I'd be making noises about it and raising the issue with the head.