score:5
For a cheap eat in spain (in general), look for the "Menu Del Dia" at lunch time; which usually come with an app, main (or main and dessert) and a glass of wine.
Near the Ramblas, there's Romesco (Carrer de Sant Pau, 28) which is cheap (it was cheap a couple of years ago).
Even if you are in Spain and want to eat Spanish food, don't forget to look at the many (turkish) kebab places where you should be able to get a good size sandwich and fries for cheap (YMMV).
If you can, try to keep bread, hard cheese and dry sausages and buy fruits and vegetables as you need them.
Good luck.
Upvote:2
If you want to eat on student budget, go eat where students eat. University canteens are one of the cheapest places to get a nutritive meal. If you are staying in Vall d'Hebron, you are near Mundet Campus and there must be some canteens there.
However, student canteens (and "menΓΊ del dia") are only common for lunch (noon) in working days. In fact, cheap and nutritive menus are marketed for students and working people in their lunch break. Therefore, they are uncommon in touristic areas, at nights or at weekends.