What did a medieval merchant earn?
About 100 pounds a year from trade.
About $24,000 a month in today's gold.
A month of that pay weighs about as much as an apple in gold.
Rich enough that the dress laws let you wear a gentleman's clothes. Your fortune would buy a small castle.
Where the money came from
Richard Whittington, the real man behind the tale of Dick Whittington and his cat, was the third son of a wealthy Gloucestershire gentry family. As a younger son he could not expect to inherit his father's estate, so he was sent to London as an apprentice to learn the mercer's trade. From about 1388 he sold silks and velvets to royalty and the nobility, and he probably also shipped English woollen cloth abroad. From 1392 to 1394 alone he sold Richard II goods worth £3,500. He also began lending money in 1388, preferring it to showing off his wealth by buying property, and later lent large sums to Richard II, Henry IV and Henry V. He was Lord Mayor of London four times, and in 1397, within days of taking office, he struck a deal by which the City bought back its rights from the king for £10,000. The dress law of 1363 shows where such men stood. A merchant with goods worth £500 might dress like a gentleman with £100 a year from land, and one worth £1,000 like an esquire with £200 a year, in silk and silver. A ploughman with less than 40 shillings of goods was held to plain blanket and russet cloth, so even the lesser merchant owned at least 250 times as much. Whittington died childless in 1423 and left £7,000 to charity.
Sources: en.wikipedia.org · rosaliegilbert.com
What a merchant's pay bought in 1350
a year's cottage rent: less than a day's pay
a hand-copied book: 3 days' pay
a year's food for one adult: a week's pay
Harlech Castle: 82 years' pay, more than a whole year of it
the ransom of the King of France: 5,000 years' pay, more than a whole year of it
Every price of 1350, with its source, is on the prices page.