School Expenses And How To Manage Them?!

Aah the new school term begins and so too do the letter's home couple of pound here for this couple of pound there for that,
it's pretty common right!

But just how do you juggle all the costs as well as running a household of 6 as well as bills ect it can very easily get crazy.

Thankfully up until now where we lived in London they didn't d a lot of school trips but those they did tended to be free or cheap enough for those you had to pay for so that was fine.

Now we've moved and already we have two residential trips coming up in the next couple weeks.
Firstly I'm nervous as they've never been on a residential trip their only time being away from myself and hubby is with Nanny and Grandad and even then I'm calling morning and night despite knowing they are safe.

so my 7 year old is off first to a place where he will sleep in dormitories, do lots of walking/hill climbing which should be fun given he cannot take his wheelchair there. they will be learning outdoors, making camp fires and toasting marshmallows for 2 days. to say I'm dreading it is an understatement he still seems so young but after a lot of consideration and extensive conversations between my hubby and I we decided that it would benefit him in not only cementing friendships but it will give him a sense of independence, and show him we have trust in him to be a well behaved boy.
he is mega excited and cant wait, and at £30 for 3 days 2 night's with accommodation and food we felt it was good for him.

Next comes the letter for lil Miss L, again 3 days 2 nights at a activity centre where she will take part in abseiling,rock climbing, zip wires, to name a few, she will also be sleeping in dormitories and building camp fires ect,
One slight difference with this trip though it's £100 of course it's got more activities so understandable but here's my query, how can school charge us £100 when the centre's website says for a 3 day 2 night course it's £23! slight price difference there!

But that said how much fun will they both have?!
To be honest I will likely be the saddo mum wiping a tear as I wave them off on the coach and will be so sad when i don't get to tuck them in at night.
Also I'm now waiting for Mr J to come home with a letter for a trip which I'm sure he will so how much will that one be?

So what age doe you think is acceptable to go on a residential trip with school?
Do you feel they're beneficial to children? and if so why?
And how do you budget for these?