What did a medieval mercenary earn?
About 7 gold florins a month for a horseman.
About $3,100 a month in today's gold.
A month of that pay weighs about as much as an AA battery in gold.
Paid monthly, in gold if you were German and in silver if you were Italian. Nobody knows why.
Where the money came from
Florence fought the Ubaldini, a landholding clan in the hills north of the city, from June 1349 to September 1350. Its cavalry was made up entirely of hired Germans and Italians, each grouped by language under a captain. Soldiers were paid by the month: the captain collected the money and paid his men at rates set by the city. In 1349 a German horseman got 522 soldi a month and an Italian 400, at 64 soldi to the gold florin. The Germans' pay was written first in gold florins, the Italians' always in silver money, and it is unclear why the Italians took silver when the florin was stable and accepted everywhere. The city itself paid for the army's food and weapons and carried them to the field. The plague made foot soldiers scarce, so their pay rose, but the horsemen's did not change at all. Horsemen could also hope for ransoms, loot and bonuses: after the capture of Montegemoli three captains got 50 florins each, and the city paid for horses injured in battle. A German horseman's monthly pay in May 1350 was worth 5,000 iron crossbow bolts. An unskilled building worker in Florence earned 10 soldi a day in 1350, so the German's month equalled more than 50 days of his pay. Loyalty was for sale too: in 1343 the German captain Werner of Urslingen took a very large sum from Bologna, then changed sides.
Sources: economics.yale.edu · en.wikipedia.org
What a mercenary's pay bought in 1350
a day of a master mason: less than a day's pay
a peasant's wool tunic: 4 days' pay
a hand-copied book: 4 weeks' pay
a squire's ransom: 15 months' pay, more than a whole year of it
a knight's full armour: 15 months' pay, more than a whole year of it
Every price of 1350, with its source, is on the prices page.