Is this the year 6 residential?
It's a really tough call, I'm organising the one for our yr6s in June. For what they get, it's actually really good value, there's no way you could arrange for them to do even half of the activities for the same price. (Ours is £260 for 5 days activity holiday inc travel)
It is also brilliant for their confidence, team building and all those things. To see shy, scared children being supported by their peers and encouraged to try is lovely and it's great for the ones doing the encouraging too. It's not part of their education, in that it won't help their SATS results, but it is good for them IMO.
That said, it is a huge amount of money for a family to find over and above their usual budget. This year we have less than half of the year group going, last it was just over 60%. Before that it would have been almost 100% so it's clear to see that families are feeling the pinch (as if we didn't know)
So for my own DC I did (and will do) whatever I can to enable them to go, but if your DD can't, she will be far from alone. The yr6s who remain in school will do fun activities and possibly a day trip that week anyway. However, if this is the kind of school that does these trips often, rather than just at the end of yr 6, I wouldn't hesitate to say no, if it would leave the rest of the family short.
Please though, make the decision early and stick to it. It's awful for the children whose parents pay the initial deposit, tell them they can go and then have to pull out because they can't make the stage payments.