On the Talk About Curing Autism (TACA) web site, there's a great article for families on low incomes who must follow gluten-free/casein-free diets. There's a sample monthly menu and shopping lists, and the article also talks about applying for food stamps and signing up for free and reduced-price school breakfasts and lunches.
The TACA people worked with the median family of four food subsidy of $396 per month and only included foods that were free of gluten, casein and soy. They followed USDA Nutrition guidelines "to make sure the diet was healthy, not merely inexpensive." Finally, they checked prices in several states to make sure they were getting a national average.
Their article is called GFCFSF Diet on Food Stamps (Yes, you can!), and it's definitely work a look.
If you follow a gluten-free/casein-free diet, you might also be interested in my list of resources for gluten-free/dairy-free diets.
Finally, even if you're not struggling financially, you might find it helpful to read How to Limit Your Gluten-Free Food Costs.
