If you think you can make and sell meals for $2.50 and make $2.00 in profit on every meal, you should go into the restaurant business.
So, adjusting for inflation, that meal would be around $8.75 today. And with todays food prices, and today's higher minimum wage, it is still profitable.
NO ONE would pay $9 for this meal today, but we were paying $2.50 for it 2 decades ago.
A microwave hamburger patty is $0.90, a bun is $0.18, a gala apple is $0.65 - these are grocery store prices, not bulk orders from suppliers. But that comes out to $1.73 vs $1.75 ($0.50 adjusted for 20 years of inflation).
EDIT: As others have stated my inflation calculation is off by about 2x, but the point still stands, I can remake the same meal today at grocery store prices for less than the school meal cost 20 years ago when they were buying for 2000-3000 meals per day. There was definitely a profit being made on those meals. Along with all the other fees we were required to pay to attend our public school.
But if you believe that you can sell any meal for 80% profit you should definitely open a restaurant.