If you are on a tight budget in Yerevan, there are plenty of places in Yerevan where you can have decent home-made food for less than 4 EUROs per person. Unlike the familiar brands like KFC or Burger King, which are also available in Yerevan, these local brands are the main places where office workers usually have their lunch. They are located in Yerevan city-center and are easily reachable. To name a few, they are My Lunch, located near Erebuni Plaza, the Aries Lunch located in the Piazza Grande, Karas restaurants, etc. They are both located near Rupublic Square.
If you have more budget to spend, we would recommend visiting one of Yerevan Tavern restaurants for more reastaurant-feel atmosphere and an a-la-carte menu. The dishes are served very fast and the taste just fantastic.