Trying to sell cheap food to chiselers is small margin, but margins are not always small. A decent restaurant can easily get B200 for something that sells for B80 at a dumpy place, and the food may be better at the dump. A nicer restaurant can get B3-400 for the same dish.
Even the cheap breakfast can make a little if you have the volume. Today at Macro:
2 Eggs: B7.3
2 Sausage: B7
2 Slices white bread: B4
200g Potatoes: B8
So B26.3 for the basics.
You have to decide if your focus is on repeat business or one offs, and what your target client is.
Generally, if you want to make any money, you either have to have great foot-traffic or plenty of parking.
Anyone looking to open a restaurant should understand that the minimum basic requirements can be studied at any McDonald's.