Wheat Prices
From 1348 to 1351 in England the price of wheat jumped around 150%.
Lower Classes
There was lots of food left unharvested and many fields left uncultivated as a result of there being no one to perform those tasks. This means that even though wages increased as the labor force died off the decrease in food supply caused the rise of, not only the price of food, but the cost of living. So while the bargaining power of the lower classes increased during and after the Black Death, because of the raised cost of living, they still remained within the grasp of feudalism. This grasp of feudalism would not be challenged until 1375 where wages remained the same but the price of food dropped.