Moany I think you would have been much better having a thread asking whether it was right that you had been asked to pay £40 to be a parent helper at a Guide camp.
As you can see from the answers above £40 for a guide to go to a 2 night Guide camp is a good deal, £40 for a parent helper is definitely not.
It would be interesting to see whether the leaders are having to pay too. I would assume the leader has badly budgeted the costs for the camp.
I don't know anything about how Guide groups are run as DS is at Scouts and I am Treasurer for his Scout group, but do you have anything similar to a Group Scout Leader (so someone who is in charge of the local Rainbows/Brownies/Guides) you could go to to query whether you have to pay for this camp.
As Treasurer I do like to keep a tight rein on our funds, but I would not expect a parent volunteer, who is helping with adult ratios, to pay for a camp. As I have said in an earlier thread we do normally ask for a contribution towards food for parents who help out on camp, but those parents are there because they want to help (and join in the fun) not because we need them for ratios. Unless your group is in dire straits there should be enough money in the pot to cover your costs.
If the Group does need more funds, especially in respect of this camp, is there time to have some fundraising activities or contact local trust funds/groups who could give a donation towards the camp. Organisations like Rotary, Lions, Roundtable regularly give us donations, especially towards a particular thing, be it a big camp or large piece of equipment. Some of these donations can be as small as £50 but every little helps, as they say.