It is possible to do some kind of trip for less than that, but for one which includes lots of activities which require equipment,N instructors and high ratios, it's a very usual price.
I agree that schools need to give loads of notice and a payment plan option and that school based activities need to be available as an alternative - and if all that happens it is okay and good to offer these things. They are very enriching and make a big difference to lots of kids - it's often when pushing themselves to try a new activity that they gain in confidence, or by staying away they develop independence. And yes, some children have done it all before with Scouts or on other holidays, but some haven't.
The reality is that this kind of enrichment does cost. Some people seem a bit out of touch with the reality of the cost of this kind of thing - they think schools are creaming off money, but schools usually work hard to get costs as low as poss. Not everyone will ever be able to afford it, but that doesn't mean it shouldn't be offered at all. Lots of families will be able to afford it given plenty of notice and a payment plan, if they choose to go for it. For others they won't need to give the cost a second thought. Some won't get to go if school can't fund some places or subsidise some. I really don't think that this means the visits shouldn't be offered at all - yes, schools should shop around for a good deal and offer plenty of notice and payment plans, but they cannot be responsible for making sure all can afford it.
I'd also mention that running a trip like this involves hours and hours of work for schools. This will all be on top of the rest of their work....loads and loads or first assessments and paper work and dealing with money, plus meeting with staff and parents and then the time away which isn't paid extra. It's not because it makes money for individuals or schools but because they believe it really is educational and makes a difference.
Sometimes I no families do need to be prepared to make some sacrifices....and before people jump on me and say they can't afford food or bills,nor course I don't mean that, and I don't mean a family should go without their one camping holiday if the year either....but I do think that given a years notice, lots of families can and do manage to make a few sacrifices and plan ahead and include thing like birthday presents or gifts in the cost and find a way. However, I also see lots of families who believe that every single thing should be paid for by schools and parents should never have to pay for a pen or transport to a swimming lesson, or a revision guide, or any kind of trip - some of these people are hard up and some aren't but feel on principle that they must complain about any cost at all. Perhaps they haven't grasped the reality of school funding and that many schools don't have enough glue tpsticks or paper and have had to sack the teaching assistants, never mind providing any extras. Unfortunatley the reality is that parents WILL be asked to pay for more and more, because schools simply can't cover it...parents can say 'no' and education will become even more basic nd Spartan and grey, or those who can afford it easily or with a bit of a push, can accept they will have to pay and that is is just one of the costs of bringing up a child. And yes,nor course it's unfair that not everyone will be able to afford it even if they scrimp and save, but perhaps you have to look at the wider school provision and most schools will have provided some enrichment for all in some form and much of what is offered is very much optional.
Finally, why are providers like PGL etc used who obviously have to cover their costs and make some money too? Because firstly it already takes hours and hours and hours of work to run a reduce tail for school staff, even when using an established company and doing it all from scratch without using such a firm would take many many hours more (which would mean most trips just couldn't happen) plus the other key thing is insurance and expertise which these official companies supply and many local authorities or academies are not prepared to endorse trips without such suppliers...because the risk is simply too high to offer such activities and organise such trios without a big name and its insurance and expertise behind it all. So often it really is, use these companies or it can't happen.