*applauds Alithea for hitting the nail squarely on the head*
The nearest thing there is to convenient healthy food for poor people (aside from raw fruit and salads, which are DAMNED expensive) is Crock-Pot stews--because a Crock-Pot is cheaper than a stove, and allows you to cook while you're not home. However, non-cooking food prep time isn't diminished at all, stews and soups get monotonous, and you still have to have that extra $25-30 to BUY a Crock-Pot in the first place. The only really fast, super-cheap thing you can make in a Crock-Pot is pea soup. I defy ANYONE to live on nothing but pea soup.
There are great pre-prepared, individually-sized salads in my grocery store, but they cost a little over $3.00 each and have less than 300 calories. I'm tiny and don't exercise much, and I still need to eat more than the salad in order to have a reasonably healthy meal.
My healthy meal is a salad (or Lean Cuisine meal--roughly the same caloric intake, but distributed differently WRT carbs, protein, and fats, and containing more sodium), a cup of Greek yogurt*, and a piece of fruit. Total cost per meal: about $4.50. Total calories: only about 500. Compare this to a Whopper meal at Burger King, which has 1300 calories and costs about $1 more. When you're doing blue-collar work, you usually need those extra calories more than someone who sits at a desk all day (like me). Unfortunately, because junk food is cheaper per calorie, this also means blue-collar workers get less in the way of vitamins.
* I'm basing this off a cost of $1 per serving, which is slightly less than the cost of an individually-sold yogurt cup, and slightly more than the cost of 1 cup out of a multi-pack (which is also usually a smaller cup). Greek yogurt has less fat and more protein than regular yogurt, but it also costs more.