What you say is right in principle, but the difference in cost between raw food ingredients and ready-to-eat meals from a supermarket is huge (ten times or more) and in comparison with the same food bought in a restaurant it is even greater, of up to 50 times or even 100 times.

Because of this great price difference, it is not very difficult to optimize how you cook in order to reduce the time spent with that down to a level that would save money even by your accounting method. Moreover, when you cook your own food the greatest advantage is that you know precisely the quality and the quantity of the ingredients that are included and that may be enough to make it worthwhile.

For instance, I normally eat only food that I cook myself from raw ingredients, but I spend little time for that. I buy some ingredients from a supermarket once per week, which requires about a half of hour. Other ingredients I order from an online shop and they arrive at my home.

Cooking requires each day about one hour, i.e. about a half hour for each of the 2 meals I eat, but most of each half hour is waiting for food to be cooked in a microwave oven or for it to cool down, when I can work or do any other activities. Only about 10 minutes are needed at each meal with things like paring and slicing vegetables, mixing ingredients and washing dishes. I bake even the bread that I eat, instead of buying it. Cooking with a microwave oven allows a lot of shortcuts in comparison with traditional methods, e.g. many dishes that require stirring when done on a stove do not require it when done in a microwave oven, which frees you to do other things. The same is true with bread, which does not require the traditional kneading of the dough when done in a microwave oven, but you can use an excess of water (or milk) that makes the dough fluid enough to mix it easily and this is sufficient and the bread will grow tremendously anyway.

Years ago, I was wasting much more time with cooking, so I typically cooked in a weekend day the food for all week and I just reheated it every day, but after gaining more experience I could reduce the required time until I could afford to cook only immediately before eating, because the time needed for that is short enough.

Nonetheless, when cooking for an entire family while having a job that occupies most of your time, probably only the method with cooking in a weekend day for all week is feasible.