It’s not just a matter of unit price, but also time, convenience, and cal/$. Fruit / veg in general is pretty expensive when looked at from a cal/$ perspective — you pay for a lot of water there. Potatoes, beans, rice are pretty slow cooking, and require dealing with dishes after.
So for my single mom, it was pretty hard to provide anything but
1) fast food (decent cal/$, extremely convenient)
2) microwaveable tv dinners (Meh cal per dollar, but long shelf life, extremely convenient)
3) PB / lunch meat sandwiches (basically god tier for poor chow)
For reference, my mom worked many jobs and got maybe 4-5 hrs sleep a night. Adding 30 minutes for a days worth of meals was a lot.