Agree with the pp to factor in locker rental and food inside the cooler costs, but for a family with 7 adults - I think the savings BY FAR outweigh those costs. Figuring here:
Week long trip - 7 days at parks:
7 x $9 rental fee = 63
$100 - $150 groceries (figuring on one meal per day - with 7 people might be tough to fit enough for 2 meals per day in the cooler, unless you rent 2 lockers) - sandwich meat, PB&J, couple bags of apples, few bags of chips, couple bags of cookies, granola bars, bread or rolls, 2 cases water, couple 12 packs of other beverages. Honestly thinking it will be much closer to $100 than $150.
So total would be $163 or on the high side $200.
Figure 7 adults x (example CS meal - Cosmic ray's burger and fries $9.99 plus drink $2.50) makes it about $13 = $91
Plus one snack per day figure $3 average snack cost x $7 = $21
That means you would spend in around one and a half days about what you would spend for a whole week of lunches/drinks/snacks kept in a cooler.
That's why we love doing it!