My understanding is that many things affected the economy in Europe at this time. There were several reasons for the decline of the feudal system: including the invention of the long bow which could piece a knight's armor from a distance; another enormous change was the ability of poor serfs to raise sheep. The cost of wool imported from the Continent was high...in England, poor people stopped working for feudal lords and began raising sheep. English wool was soon preferred. Castles became obsolete.
The Black Death (plague) killed so many people that work would have been easier to find with a reduction in population.
The plague (and any disease) kills the old and weak more quickly, but the Black Death knew no boundaries: kings were as susceptible as peasants because they had no way of knowing how to stop the spread of the disease. Pohnpei397 hits it on the head: when there were many people looking for jobs, merchants, etc., could afford to pay lower wages. With the shortage of workers, wages had to be competitive—higher. Anytime a business wants to make a profit, it has to find ways to decrease costs. Technological advances would have helped, even if they were archaic compared to what we would see developed during the Industrial Revolution hundreds of years later.