> I was using USDA figures for median food costs

Median people in the USA eat extremely lavishly. Your series of comments, to me, boils down to a complaint that you can't maintain this extreme luxury while having children. Yeah, nobody ever could. The time frame in which people were "able" to have a lot of children was the time when they weren't living such extravagant lives of consumption. So yeah if the complaint is "Why can't I afford a lifestyle that would make my grandparents blush with shame at its luxury, and have as many kids as them, wtf?!" then I'm not very sympathetic to it.