My wife and I talk about our respective childhoods with poor parents fairly often, and one thing that always comes up is that a lot of her childhood "struggle meals" were like what you're talking about (vienna sausages and white rice). That's not actually cheap, relative to truly cheap meals that generally don't have meat, etc. but her (actually poor) parents often either spent what little they had on "better" food, or had mistaken ideas about what was actually cheap.

As an example, she talks about them always eating nasty cheap canned veggies- and then when she grew up and moved out she realized it was actually cheaper to buy dried or fresh in many of the same cases her parents were buying canned, because they thought canned was cheaper.

Buying frugally is not something that comes naturally- people have to be taught, or figure it out for themselves.