I have to disagree with at least one of your statements. As a former general manager of a fast food restaurant. I know personally how much money was spent to serve every customer, because my monthly bonus depended on how tight a rein I kept in my store.
It cost my store to sell you a medium drink less than a small or a large. We are talking a difference of 5 cents either way. But it was more profitable to sell you a medium than a large or a small drink.. Breakfast was the money maker. Burgers were a loss leader. But everything was an add on. That slice of American cheese cost less than 1 packet of ketchup. In the early 90's it cost 3 cents for that packet of ketchup. And that is why you always need to ask for condiments, let alone extras. We charged the customer 25 cents for the privilege of adding a slice of cheese to any sandwich.
My profit margin was never affected by the price of anything supplied to the store. That contract was set in stone.
My profit margin was affected by employees. My little babies, who ranged from 14 to 18 were the best. That group always showed up. Their parents showed up to help me secure the store for hurricane Andrew. The older employees that couldn't find anything better were the ones who stole, showed up late, showed up hungover, didn't show for days and thought had a job. The older employees affected my profit margin. And not one of the older employees showed up to help me secure the store for the hurricane.
My profit margin was huge. I made big money as a FF GM with my monthly profit bonuses. I made 80K with profit bonuses and that was a bad year. It was also in the early 90's.
Why don't I do it anymore? Honestly, except for my little kids; the job was a huge PITA and required a solid 80 hours a week with all the paperwork, plus actually building the burgers. Usually my week was more like 150 hours; when I finally quit I was down to 82lbs. I'm only 5'3" tall,. I was a walking skeleton with a personality to match. Plus there is nothing you can shower with to get rid of the smell of burger in your skin on your only one and day off a week. LOL
Live your life in such a way that when your feet hit the floor in the morning; the devil shudders...And yells OH #%*+! SHE'S AWAKE!