[JackMonroe]

I don't see how a grocery store is cheaper... I can get a full meal at McDonald's for $3.50 - that will get me a box of Cheez-its at a grocery store.

An entire box of cheez-its is more than a full meal.  That said, cheez-its are not quite what I had in mind.  Buy the basics.  Meat, vegetables, ect.  Don't go in there buying bags of Doritos and shit.  If you want to get really into it, factor in health costs from a poor diet.