Compare packed lunch cost against school lunch.
Packing lunches is cheaper on ingredients and more expensive in time, and the honest comparison prices both. Ingredients scale with the number of children, but preparation time largely does not — making two lunches takes barely longer than one, which is why packing gets relatively cheaper with each additional child. On these defaults, ingredients save 66 dollars a month while the prep time costs 32, leaving a net 34. Set the time value to zero if the prep happens in time you would not otherwise use, and the saving doubles. The other consideration is not financial: school meals are often nutritionally regulated in a way a packed lunch is not.
School Lunch Cost
School lunch = days x children x price; packed = days x children x ingredients + days x prep minutes / 60 x time value; saving = school - packed
On ingredients, clearly — typically 40-50% less per meal. Once prep time is priced the gap narrows, and it narrows most for a single child, where the fixed time is spread over one lunch.
Yes, because the preparation time barely changes while the ingredient saving multiplies. Two children roughly doubles the saving without doubling the effort.