I will second everything tinuviel_k said, and add that not only fast food from McDonald's, etc. is cheaper than good food. Most grocery stores, especially those in poor neighborhoods, have vast aisles full of processed foods, frozen pre-made foods, soda and other sugary drinks, but very small produce sections. The produce is very limited, usually the most boring veggies and fruit you can think of, and not very fresh. It is also a lot more expensive than the junk food in the rest of the store.
I am currently taking an Environmental and Preventative health class at university, and we watched a documentary about health disparities (which was all very shocking, even for someone who follows this kind of thing). In part of it, they followed a family of 4, a mother and father, and two teenage children, to the grocery store. The mother picked up a frozen lasagna and said, "I can feed all four of us with this, and it's only $2.50. I can't buy the ingredients to make it myself for twice that amount. A bag of veggies costs the same. I can only afford to feed my family this type of food because our food budget is only $200/ month." I have a family of four, with two elementary aged kids, and we spend about $1000/ month on groceries, and eat out at least once a week. We buy organic, local produce and very little processed foods, what we do buy is organic and healthy. It made me feel very lucky indeed that we can afford to eat well, though we certainly make sacrifices in other departments so we can do so, as we are lower middle class, living in a very expensive town.
We live in a "ghetto" neighborhood, where we bought our first house 2 years ago. There are only a couple of small grocery stores anywhere near us, and they definitely meet the description I gave above. We drive to nicer parts of town to get healthy food. Almost all the restaurants in the neighborhood are fast food places. At least half of the neighborhood kids are fat- not a little pudgy, but fat. Since they are Hispanic and African American, they have about a 1 in 2 chance of developing type II diabetes.
Our school lunches are no better than fast food places. They typically consist of greasy pizza, fried chicken, chicken nuggets, nachos, and the like. The required fruit serving is often met with canned peaches drenched in sugary syrup, and at the school where my mother taught they would sometimes "meet" it with gummy bears containing 10% fruit juice. This in a poverty stricken school where most of the children have metal caps on many if not all of their teeth. Vegetable servings are met pretty much every day by a pile of french fries. They are required to serve milk to every kid (even in largely African-American schools, where up to 80% of the children are lactose intolerant), and most kids choose chocolate milk which contains as much high fructose corn syrup as a can of soda. The milk requirement is entirely due to the huge subsidies we pay to dairy farmers to overproduce milk, which we buy and serve at schools.
My daughters go to a very progressive public school that has tried to change the lunch situation, but has found it to be so far impossible, except that they have stopped serving chocolate milk. It's impossible for a number of reasons:
1. The "kitchen" is not equipped for cooking. It has a couple of warming ovens, designed to warm large trays of the pre-made food that gets delivered by the district daily, but not designed to actually cook food. There are no stoves, and very little prep area, as everything comes in packages, ready to be dumped into the metal serving trays. There are no dishwashers and only one sink, necessitating the use of styrofoam trays and making it difficult to wash even cooking utensils, pots, etc.
2. There are extremely strict, but very strange federal requirements for a "reimbursable" lunch (one that the federal government will pay for). It has been designed not by nutritionists or chefs, but by the agencies that give out the farm subsidies that make corn, soy, and wheat so cheap, and are lobbied heavily by agribusiness and the food industry. These requirements allow some of the idiocies I mentioned above, but makes something like a chicken stir fry with veggies not reimbursable. To see this in action, check out Jamie Oliver's show, Food Revolution, where he tries to redesign a district's lunch program. Be prepared to be amazed and incredibly frustrated watching it.
3. Cost. This is huge. The school has tried to arrange catering with a local restaurant that does a lot of great work with the school (like a free gourmet, healthy cooking class after school for middle-school kids, and sponsoring events promoting healthy eating), which would pretty much just break even for the restaurant, but the cost is still too prohibitive. The school will only get reimbursed for something like $1.68 per lunch, and a decent lunch would cost about $3. For the middle and upper class families this would be ok, but for the lower income kids that qualify for free or reduced price lunch and breakfast, this is not feasible, and there are not enough of the higher income kids that can commit to eating daily lunches to cover the cost of the lower income kids.
So here we are, in one of the richest countries in the world, and we can't feed our kids decent food at our schools. The poor here are getting poorer (as are the middle class), and when people get poorer their health gets worse. We have the worst health disparities in the developed world, and they are getting worse. Food is just one part of this, of course, but as anyone with any sense knows, a good diet is the foundation of a healthy life.