I never understood how can eating at McDonald's be cheaper than cooking your own meals. I'm not from US, but checking US grocery shops, you can eat meals made of chicken breast, bread and vegetables well below 5$ per person, well below 20$ in total for a family of 4. Yet every time I see those discussions, fast food is always presented as a cheaper option?
I'm not sure where you're getting numbers but I can't buy just chicken breast for two people for less than $5 at my grocery store unless I buy in bulk. And I do not have the space to store it.
Add some simple mashed potatoes and you're still below 5$ to feed two people in one meal.
You can also eat bens, rice, lentils, eggs, add some cheese. There's countless simple, cheap, non processed food around.
The reality is that it's "more convenient", or at least it was, because if you had to choose between spending 3$ for a complete meal you still had to cook, and some 5/6$ McDonald's processed tasty food, you'd go with #2.
But stating that it's cheaper because of "scale economy" is just false, it isn't and never was to eat out. Let alone the impact of eating such junk food.