Earlier quoted context omitted.
https://biketips.com/calories-burned-biking/ says I burn ~600kcal to ride 20km. 600kcal is ~200g of bread + 20g of butter. Bread costs about 5€/kg, butter costs 2€ / 250g so 600kcal is roughly 1,25€. My car uses about 6l gasoline per 100km, current price is ~1,60€ per liter, so 20km need 1,90€ of gasoline. So if I just eat cheap food riding a bike is cheaper. If I eat at McDonalds, then 600kcal is a hamburger with me…
I think bicycle calorie burn calculators are basically just nonsense. 600 calories is a ton to burn for a relatively short leisurely ride. Maybe 5x too much. I have an older garmin that has told me I have burned 9000 calories in a weekend, and uh, I didn’t. No evidence for that, but I simply don’t believe it. Bicycle tourists would have to have wild food intake to sustain that for weeks.
It depends a lot on the bike and on the route. On a road bike going down a straight, flat road wearing lycra 20km/h is zero effort. On a city bike loaded with groceries going through urban environment with a bunch of traffic lights 20km/h average is quite an effort.