The place look like an old communism place to eat, but the meals there are great and for very low prices. I think the best place to eat Czech (best choice is to try Goulash soup + Cream souce with dumplings and beef meat) and you will pay for the soup, main meal and 1 beer about only 6 USD :)
Adress:
Havelská 21
110 00 Praha
gmaps: goo.gl/maps/En8JX
closest station: Můstek (metro station of green "A" and yellow "B" lines)