The ingredients of food are nearly free, the US average income per capita buys 700+ kg of wheat per day. The distribution/availability problem is political.
Wheat doesn't have proteins and fats - those are more expensive. And processing wheat is fairly laborious, you'd have been better off comparing it to flour (about 12kg of flour a day for the median US income after taxes). And there is also the other costs of living - like shelter (more costly if you want shelter with access to jobs).
Food costs can be quite cheap and still be nutritious with the right choices, but not 700kg of wheat a day cheap.
