To find cheap and good quality/price food you'll have two solutions: the students restaurants and students cafeteria.
Students Restaurants aka Crous Restaurants:
This is of course the cheapest and the healthiest way to have a meal.
For only 3.25€ * you'll have: The first Course + Course + Dessert
To let you know you to go this this is the list of all Crous Restaurant in Paris
This is the normal timetable of crous restaurant from September to June. To be sure that it's really open always check Crous Paris' website and chose your restaurant!
* prices for September 2018. Check the Crous Website for more info.
Students Cafeteria and Snacks Bar:
Not as good as the real restaurant and also not as healthy as the preview's one, the cafeterias as the advantage to be open during longer opening hour. So if you cannot go to a restaurant, cafeterias will provide you cheaper food than fast food. Here is the list of cafeterias:
Make sure your cafeterias will be open by checking the Crous of Paris website ;).
Note: This is the usual timetable of opening from September to June, if you are in visit during the summer visit Crous-Paris.fr first to see what's open!