It depends on whether the school has assessors, or are buying in assessors.
If the school can assess them itself, then you are only paying the cost of running it, the tents, the minibus, etc
It the school is buying in assessors then the cost is far higher, and there should be options for families that cant meet the cost.
For a large school to run a large DofE kohort, they will need around 20 staff. That will normally be 10 outside assessors, and 10 from the school.
If the school runs it themselves, they need 20 volunteers to give up multiple weekends. They also need many of these staff to be fully trained assessors - more time given up voluntarily. They also leave themselves with no wiggle room if there are any disputes, or other problems with assessors, as with an outside company, you can just change assessor
You are also left with the uncomfortable situation of students having to return into lessons for several years, with staff who have failed them
It works out best to have outside assessors, and the school staff to be left in a position where they are fully supporting, and not giving judgements.
But please be aware, many outside assessors are also doing it for very little, often only for expenses, (which of course can run into hundreds, if they have travelled a long way and need local accommodation, etc)