The New York Times breaks down the cost of two home-cooked meals, relative to McDonald's. It's no surprise the homemade stuff is healthier, but as this graphic shows, it's also cheaper. Sometimes a lot cheaper.
Of course, this doesn't factor in the cost of time. And it doesn't really deal with the fact that the up-front costs for a cart of groceries — assuming you have transportation to get to a store and time to shop in — are a lot higher than for an individual meal. (You can't actually buy 5 cents' worth of salt, or two cups of rice.) But it's a nice illustration of the fact that it is possible to eat well on a budget, even if challenges remain.