Bar mleczny, a "milk bar" is a good, cheap place to eat. During communism, the Polish government subsidized these working class cafeterias. No more communism but the prices are still low which is fine by me. 
The prices are low and the food is tasty and filling. Everything is in Polish but with enough sign language you can order a good lunch. The only thing that would make it better is if you could order some Żubrówka.
I grabbed some borscht (beetroot soup), cabbage and carrot salad, and some pierogi. Way cheap!
Pierogi are dumplings that are boiled, baked or fried. There are all different kinds. Some are stuffed with potato, meat, cabbage, spinach, or fruit. I love me some pierogi! I wish they had something similar in the Czech Republic.