I'm pretty sure this is wrong actually...
I've lost count of the number of rows I've had about that, many in public, one live on telly, during which the broadcaster Jonathan Maitland said this was a problem of education: if she knew how to shop better she would have been able to afford a more balanced diet. This is total tripe (which, although famously cheap because it is famously disgusting, is still probably more expensive than a Big Mac); certainly, you could shop differently with the same money and win the approval of nutritionist Gillian McKeith, but if your aim is to avoid being hungry, you could not do that more cheaply.
Well, you can get enough tripe for 3 people for £3.50 so that particular argument doesn't stand up. But more generally, as the commenters to Tim's piece note, if you go for supermarket value ranges of things like pasta, bacon, onion and so on, you can get a solid healthy meal for 4 at very little more than 50p a person. But even if that gets boring, you can make really remarkably good meals for not much more money - certainly less than processed ready meals cost. For the sake of the argument, lets stick with the Big Mac Meal that Zoe refers to as the cheap calorie mecca. £4.59 gets you one Big Mac Meal.
So. £4.59 per serving. I'm a flash bastard, so I do my shopping at Waitrose, making this something of a handicap event as far as I'm concerned, but lets see if I can't beat that. My current favourite 'it's cold I want something comforting' meal is slow cooked ox cheek. How much is that? 500g of ox cheek sets you back about £3, and makes enough for 4. A pint of stock, since we're being frugal, comes from a stock cube, from a box of 6 for 75p. A red chilli, from a bag of four for 60p. Some garlic, at 52p for a whole one. A tablespoon of flour, from a bag costing 48p for 500g. (If we're pushing the boat out, we can add a slug of sherry, at £5 a bottle, but you don't need to). To go with it, some rice (£1.62 for a kilo), and some spring greens for 99p. Where does that leave us? £8 if we're being frugal, £13 if we're splasing out. In other words either £2 or £3.25 per serving - and we'll have lots of stock cubes, garlic, chilli, flour, sherry and rice left over for next time.
What makes the Big Mac Meal more attractive is not its price. It is its convenience. The meal set out above takes about half an hour to prepare, and about 3 hours to cook. That's fine by me, because I enjoy cooking and at weekends I will almost always have half an hour in the kitchen where I can concentrate. That's genuinely not fine for a lot of people, who have neither the knowledge to prepare the meal, nor the time to cook it. Zoe Williams is right that it's poverty that causes poor diet, but it's poverty of time and poverty of education in equal measures, and not poverty of money.