Cooking would probably still end up being cheaper in most cases but not by a huge margin. But then again Paris might not be very typical in that regard, food is notoriously expensive here.
Given that most delivery services have a minimum, it's pretty difficult to spend less than $15-20 to get your food delivered.
In my experience, it's about $2-4/meal to cook, $12-15/meal to eat out, and $20+/meal to get delivery. Imagine that over a year. If you're cooking at $4/meal vs eating out at $12/meal, that's ~$3000 over a year!
On the other hand, you are paying for convenience and some people are willing (and able) to shoulder that cost.
A game changer for me has been a high quality rice cooker. Put rice or quinoa in the cooker as soon as I wake up and my at my “lunch break” I prepare some other stuff, eat and get back to work. I work from home all day generally.
What?!?!
I get the feeling your definition of "not a lot more expensive" is a lot more inclusive than mine.
Obviously cost of home cooking vs prepared depends a lot on where you'd eat out and what you'd make for yourself but in my experience fast food off the value menu is about the only thing that's close to cost competitive with cooking for yourself. I would have to eat almost exclusively organic and add a lot of wild non-frozen seafood to my diet if I wanted to make the cost of preparing my own meals close to the cost of buying Chipotle/Five Guys quality prepared food. Maybe the numbers are different for people who shop at Whole Foods or otherwise go out of their way to increase the cost of groceries but your statement that eating out is close to cost competitive with cooking for yourself seems like it's from another universe.