What did a medieval serf earn?
Paid mostly in grain, plus a few coins twice a year.
About $300 a month in today's gold.
A month of that pay weighs about as much as 19 coffee beans in gold.
You plough the lord's fields for sacks of grain and see coins twice a year. Cash is a rumour.

Where the money came from
A manor's ploughman was one of the lord's year-round farmhands, hired most often for the whole farming year to plough and to herd the animals. Most such hands were not free in law, and they stayed unfree to the end of the 1300s. Until the 1370s only about a fifth of their pay came in coin. The rest was a fixed measure of grain, which kept them fed whatever the harvest and whatever grain cost. A year's grain weighed between 567 and 1,248 kilograms, so leaving for another master meant giving it up or hauling it away. Small extras came on top. At Barnhorn in Sussex in 1353–54 each ploughman got 2½ pence twice a year, at Christmas and Easter, to give as an offering in church. On some manors the hands also ate at the lord's table, with bread, meat, fish, cheese, eggs and ale. Before the plague the lord's accounts listed them by their job alone, never by name. One historian called these year-round hands the aristocracy of labour, because their work was secure. Yet their pay, about 25 shillings a year on average, took 80 years to double, while a day labourer's doubled in a single year after the plague. The ploughman had the safer place, but the rise a day labourer got in one year took the ploughman eighty.
Sources: vincentdelabastita.github.io
What a serf's pay bought in 1350
a dozen eggs: less than a day's pay
a hen: 5 days' pay
a plough: 3 weeks' pay
a year's food for one adult: 19 months' pay, more than a whole year of it
a one-bay cottage: 19 months' pay, more than a whole year of it
Every price of 1350, with its source, is on the prices page.