I live in an area with lots of, in theory, better off parents.
In practice they are mortgaged up to the hilt and actually have surprisingly little disposable cash.
The school have made a point of keeping trips much cheaper than that. £600 is a lot of money for virtually any family to find right now regardless of whether from the outside they appear wealthy or not.
As a side to this, DH does a lot of community work here and every year the schools have requests for emergency Christmas Boxes. We don't know who it is, the schools keep it confidential, but we do know it's from parents who fall in between the cracks and often have had an unexpected personal issue (eg having to take unpaid leave at short notice for an illness or an issue with a family member). They aren't eligible for benefits, look well off but are financially very insecure.
Ironically we do know people living on the local council estate who live on benefits who have more disposable cash than some of these families. And of course quality for assistance with these type of trips.
It is causing a certain amount of friction within my circle of friends at the moment as the council estate party makes a song and dance about how poor she is and then splashes cash about in a way some of the parents who are firmly in the middle just can't.
There's this weird taboo in this middle group about admitting how much they are struggling. It really needs them to be vocal that actually the school is out of touch with the local community and is being massively insensitive and taking the piss. It only serves to highlight the tension above about those who are eligible for assistance.
Honestly you won't be the only one in this position. The school are being really unfair.
The school are not grasping that a family with two or three kids won't just be paying this once either.
Don't just ask for assistance, frankly complain.