A pound of rice goes for 10-15 cordobas a pound. Red beans are expensive at the moment as said before. They go between the 19-27 cordobas a pound. I found the cheapest ones at Pali (supermarket owned by walmart). But any local Managuan will know where you can get the cheapest things. And you help the local economy if you buy it at the market. The market is also the best place to buy cheap oil, body-soap, consume, cookies, etc….I find pricemart expensive if you don’t need anything American.
Managua has several markets. El Mercado Oriental is the cheapest they say, and most dangerous for foreigners. Other markets (like Roberto Huembes) are more safe but I would definitely send a list with a trustful Nicaraguan so you are sure you get the best price. I normally go to the market and if I need something I don’t know the price of my husband will buy the stuff and I will go somewhere else.