This is a hard question to answer, because gardening allows you to have fancy food that a Mustachian might not otherwise buy in the store. For instance, I wouldn't pay $4 for a bunch of fresh basil, but it's economical to throw down a few seeds and have a never ending supply of fresh basil + pesto.
But with that said:
Spaghetti squash. I grew one plant last year and got probably 25 squash off of it. Still have several in storage.
Also agree with potatoes. I don't hill mine, and also have plants growing from spuds I missed when harvesting in prior years, so other than water, $ + work investment = 0.
In my house in California, the pecan tree was pretty awesome. Little maintenance, tons of delicious pecans, which are pricey and packed with calories.
Zucchini, because it is notoriously prolific, and also super versatile, can be used as a meat stretcher, hidden in bread and so on. Some varieties of squash work as both summer and winter squash, making one plant even more useful.
Garlic, because in theory, once you have seed garlic, you never need to buy it again.