The cheapest way to eat in a restaurant is to go to a "Menu del Dia" Restaurant.
This is a fixed price menu with 3 set courses.
Prices start at around 8.50 euros for a 3 course meal.
The 3 courses are:
Starters - Primer plato or Entrada
Main course - Segundo plato or Plato principal
Desert - Postre
If you don't speak Spanish a "menu del dia" may be a little difficult because many times the menu is not written down and the waiter will list all the available dishes very quicky and you will be confused.
Try to read the menu on the wall before the waiter comes.
If there is no printed menu hopefully the waiter will speak some English.
Otherwise just go for pot luck.REMEMBER
The word "Menu" in Spanish always refers to a fixed price menu.
The word Menu in English is "la Carta" in Spanish.
So if you ask for the "menu" the waiter will normally think you want the fixed price menu.
If you actually do want to see the menu with all the dishes available then ask for "la Carta"
There is a list of cheap restaurants on this pagehttp://granadamap.com/places/cheap_restaurants_in_granada.htm
The Girasoles is the cheapest place that I know.
If you want ultra ultra cheap go and eat in the dining room "comedor" at any of the university faculties.
Many years ago I used to be an expert on the university dining rooms.
The best one was "empresariales" (The business faculty) They even had table cloths!!!!
The "comedor" at the Science Faculty is the easiest one to get to from the centre.