A lot of industrial produced food is cheap because of child, forced, and otherwise exploited labor (undocumented workers, for example). Heavily mechanized farming (mostly used for grains) is cheap because of the vast amount of fossil fuel "energy slaves" used. And that's only cheap because the costs are externalized.

Anyways, growing your own food can definitely be cheaper than buying it. Of course, not if you start plants under lights, build raised beds and fill them with purchased soil, buy organic pelletized fertilizer, or stuff like that. It can be nearly free to grow your own food (if you don't count the cost of your own labor) by saving seeds and intercepting materials from waste streams (wood chips, lawn clippings, manure, used coffee grounds, etc) to "feed your soil."