Right now where I live there are 5-10 McDonalds I could visit within a 30 minute round trip time. The cheapest burger (just a burger) is 400 calories and $1.39. $7-$8 is about right for a meal, but being fair one of those has more than half the calories a person needs in a day, and it would be possible to live off only that once a day for a long time.

I could make a healthier, better tasting meal for $1.39 in ingredients but that would be quite restrictive. I don't know that I could make a burger. More importantly though I would have to make sure the perishable ingredients were fully used before they went off, I would have to have kitchen equipment and knowhow, and I would have to have the time and energy to make this food. Some of those things are luxuries not everybody has (even though they're quite a bit better financially, healthwise, etc. in the long term)

When I cook for myself realistically, I usually spend more than I would have at McDonalds (not counting whatever my time is worth and kitchen equipment cost).

Could I eat cheaper than McDonalds without cooking myself? Not really, unless we're talking about a few cents.

Could I eat cheaper than McDonalds by cooking myself? It depends on my choices but generally yes, but requiring some careful overhead management.