Well I was trying to figure out how much we spent when the boys were in their teens and my dad and daughter were here. At least 2 times a month at Sam's/Costco with the tab running around $250 a pop, (of course that would include some non-food items). 3-4 trips to local grocery an average of perhaps $30.
250 X 2= $500 4 X 30=120 X 4= 480 500 + 480=980 divided by 4=$265 per week
Not counting eating out, pretty close to $300.
Now however, we eat pretty much veggies, fruits, whole grains, chicken/fish twice a week. Oatmeal for most breakfasts. I'm sure we're spending considerably less than a quarter of what I did.
I will say though that with teenaged boys, it's not possible to keep food in the house. there were probably 5 really crazy years, where we went through 3 gallons of milk, 4 or even 5 loaves of bread, countless boxes of mac & cheese, most of these not at meals, but 'to get them through'.
I'd say in the long run boys are cheaper than girls, but not in those few years where they go from 4' something to close to 6' and from maybe 95lbs to 150 or so. It'd be cheaper to fatten a calf and at least you could it 'em when you were done.