Each day in Canada we ate, on average:
Porridge: a cup of oats, 150ml of milk
Sandwiches: two rounds of peanut butter and jam
Dinner: two eggs, rice
Snacks: a few handfuls of nuts, a couple of apples
I’d consider this the bare minimum in terms of acceptable amounts of flavour and how inspiring the food is.
At Aldi it will cost you £2.61
(Almost definitely be cheaper still in an Aldi store; they have a weird and limited selection on the website).
At Waitrose it will cost you £2.43
That works out as ~£18 per person for food that, while not particularly inspiring, will keep you full and caloried up on an adventure.
The cost increases as you add tasty extras like coffee, chilli sauce, seasoning, beer, cream for porridge, and if you’re feeling extravagant, beans in the rice. And the longer you’re on the road the more tempting these things become – as do pizzas, sweeties, more beer, and so on.
But the base cost for a week of food can be kept lower than you might expect.
Although be aware that at rural general stores and village convenience stores, the prices will creep up as you get further away from civilisation.