Cheapest way to eat without dying?

My little brother got a survival kit and it came with three days worth of food rations (mayday bars). This got me thinking, If I just wanted to eat the cheapest thing I can eat for the rest of my life, what would that be. Whats the most healthy calories I can get out of my dollar? Out of everything at the supermarket what's the cheapest, healthiest and most filling thing I can consume a lot of without dying, staying away from High fructose corn syrup and all that bad stuff, of course (which is really cheep). Is it safe to just eat ration bars your whole life? I do not see food as an indulgence and don't care about flavor as long as I can tolerate it.