Like everyone else, activity centres and accommodation for school trips are facing higher bills this year- and so they have had to put their prices up. I've seen a few primary headteachers talking about this on twitter recently- prices have gone up, and some are looking for cheaper options.
But that's not an easy option either- parents often expect one particular trip because it is "tradition", so switching to a cheaper option can be controversial.
You say trips shouldn't run if parents can't afford them, but for some people £100, £150 etc will be unaffordable. And I don't think there will be many residential options at this sort of price available for summer 2022, either. So, then realistically, you get no trip.
If the school already pay for transport via fundraising, could you suggest additional fundraising events to bring the cost of the trip down? I work in secondary, and have seen this sort of thing done in the past- obviously older students but they raised money towards expensive trips by doing things like car washes, quiz nights, raffles, and so on.
The money raised could be distributed evenly- bringing the cost of the trip down for all, or the school could allocated it as needed to families who can't afford the trip at all.
I do agree it's a really difficult decision as to whether to run these sorts of trips at all in the current financial climate