5 days activities, 4 nights accommodation and food.
A day at legoland - wander round and go on some mechanical rides - costs £47. Plus your travel costs to get there, plus car parking plus food.
On a PGL trip they will presumably do loads of activities which are in themselves expensive to run. Kayaking, abseiling or whatever. Staffing the coach trip, staffing the activities. (A session at Go Ape costs £33, for example.), staffing the accommodation (a full board night in a youth hostel is £31, for comparison).
So:
Coach trip and travel while away - say £50 pp
8 activities (1 on Mon and Fri, 2 each on Tue, Wed, Thu) x £30 = £240
4 nights accommodation and food = £120
Which takes you to £410 before you cover Teacher supervision, insurance, evening entertainments, equipment hire if they are loaning the kids boots, cagoules, rucksacs etc, and any other overheads and expenses.
I'm not saying it isn't a lot of money, but I don't think it's a rip off.