This blog post
talks about how she calculated the expected amount for food for her family. It links to the official USDA average costs for a family of four to eat. I can't put two links into one post as we all know, but to sum it up, the expected cost for a family of four runs from $533.40 to $1208.10, depending on the age of the children and which plan you think is "reasonable."
Wow that is a really useful blog post. Estimating for the family closest to us, our food costs (ranging from thrifty, low cost, moderate, liberal) are: 611, 796, 995, 1208, and that is assuming all food gets eaten in the home. If we don't include restaurant costs we are not in the highest group! I'm trying to see whether 800 a month grocery (also includes incidentals) and 200 a month restaurant is doable.