For veggies, I would say that for some things I fairly easily do better cost-wise than I would paying for the same items at an organic grocery store or farmers market. Of course they charge something like $3.99/lb at the farmers markets here for heirloom tomatoes. Garlic, onions, potatoes, sweet potatoes and some root vegetables like radishes, beets and carrots all do well for me, but they are usually so cheap at the store, I’m not sure I’m saving much money on those. But I grow varieties I like better and I may be saving a few cents. If you like red onions, I had a great crop of Cabernet F1 tomatoes this year. Here are a few I entered in our county fair. Don’t be too impressed by the blue ribbon, since I live in an urban/suburban area and there are very few entries for most categories, lol. So far they’ve been keeping really well.
I like things that give me two crops like garlic (scapes and heads), sweet potatoes (greens throughout the summer and the potatoes in fall) and beets (greens and roots) and I also consider the timing of each to try to get multiple crops per bed. So when the peas are done in spring for instance, they’re replaced by beans. I actually plant the peas at least a foot in front of the trellis and train them back to the trellis, so when they’re done, I cut them off at the ground to leave their nitrogen-rich roots in place and plant peppers about 18" in front of the trellis to use the nitrogen and plant the beans back at the trellis so there isn’t so much root interference. The trellis runs east west, so there isn’t much shading to worry about, but even if there was peppers are happy to have a bit of shade in our summer heat.
I think Malabar spinach is a great suggestion and I see a lot of people growing that in our community gardens here and it is vigorous and seems to love the summer heat. You could grow a colder season crop like lettuce, kale or chard in the cooler months and then go to the Malabar spinach when it gets hot. I use the sweet potato greens in the same way, so haven’t bothered to try it.
Overall, greens are far and away the best return. If you grow from seed, and use good soil management practices so you don’t have to constantly add expensive fertilizers, you’re basically getting a head of lettuce for a few pennies. Same with kale, etc. And since most can be grown as cut and come again, you can have a long and predictable harvest window.
If you serious about saving money, I would highly recommend starting your own seedlings. I find that even with lettuce and other greens, I get better spacing and production from starting them inside. Then you can get a weekly crop of microgreens from your seed starting setup when it isn’t needed for seedlings. These are just starting to open their leaves so need another 2-3 days before harvesting. Since they are living, you just clip what you need when you need it and if they’re getting too big, cut them all off and put them in a ziplock in the fridge. They’ll keep at least a week.