If you go by calories, then you'd probably have to say nuts are the cheapest things out there
For me, I know I spend less eating healthier as my grocery bill has gone down even though I eat a bunch of organics. Right now I'm at the point where most of my produce is organic but its also summer, winter may change things. I buy a lot of basics, veggies/fruits that are in season/on sale, bulk nuts, legumes, grains and spices. I rarely buy anything that comes in a package and when I do, I expect to pay more. Overall, my grocery bill is between $200-$300 month for 2 people. Lately, I've been closer to $300 right now and looking to cut it down a bit.