I would probably say that the combination of the long inflation and population recovery after the black death.

The influx of gold and silver from the new world into the economies of the old world caused a long inflation of about 1-2% yearly for about a hundred years. Barter economy was replaced with a coin based economy as the value of silver coins fell and they became much more widely distributed.

After the Black Death in the mid-14th century, there was a distinct lack of Labour and wages rose sharply as a result - population recovery after this would slowly increase the supply of labour and drive down the price of it.