Vietnamese bar very close to the Banach Center. You can have pretty good food for fairly low price.
Salad bar "Takan".
The restaurant of the Warsaw High School of Gastronomy. Low prices of food (they exercise on you). The lunch service starts at 13:00. Closed during weekend.
Mini-restaurant "Forte". You can have really good lunch for about 22 zloty.
Bar-restaurant "Piano". You can have good lunch for about 20 zloty. The grilled porc can be recommended.
If you want to meet the atmosphere of a Warsaw flat before the War, try "Przysmak". About 10 people are packed in a small dining room, where a very good, reasonably priced, home-made food is served. The entrance through the yard. Closed during weekend.