We seem to have got very cheaply out of this:
trips- dcs only go if we feel we can afford them
out school uniform is actually cheaper than most other clothes (couldn't have them walking around naked if home educated), lasts for a longer time and easier to find in charity shops
as for charity, I would give to charity anyway, it's nothing to do with what the school wants, though I don't mind it being managed by the school: again, I decide how much I can afford to give
packed lunches- well, again, they'd have to eat wherever they were, and a sandwich to take to school is not more expensive than a sandwich eaten at home (use old margarine tubs for lunchboxes)
and the hours they spend at school are hours saved from childcare expenses seeing that both dh and I work
I suspect extra expenses come a lot cheaper if you live in an area where many people are short of cash, so social expectations are not enormous