In Ulaan Bataar there are thousands of kiosks along the roads, who sell all kinds of stuff, not too expensive. These kiosks are mostly open 24 hours a day as the shopkeeper is preventing his kiosk from burglary. These shops can fairly well compete with the larger grocery stores.
The grocery stores are working, however, in the same way. A collection of kiosks in a large building. Near the 13th microdistrict is Durvun Uul, or red and white building. If you learn how to cook sheepmeat, potatoes and cabbage, you can have a very cheap time. Products well known in western countries have much the same price as in western countries. Milk sold in bags is cheap; milk sold in cartons is expensive. Mongolian advice: don't buy airag (fermented horsemilk) in UB. It is often mixed with water or cowmilk which gives troubles to your health.