What did a medieval man-at-arms earn?
12 pennies a day, half a knight's wage.
About $4,400 a month in today's gold.
A month of that pay weighs about as much as 9 dice in gold.
Full armour and a warhorse you buy yourself, at a hundred days' pay. Pray the war lasts.
Where the money came from
A man-at-arms was a horseman in full armour, and the name described his job in battle, not his rank. Every knight who went to war served as one, but most men-at-arms were not knights. Many were squires from knightly families, rich enough to afford a knight's arms, and some avoided the title because they did not want its costs and duties. Between the 1280s and the 1360s knights usually made up only 20 to 30 per cent of them. In the mid-1340s a knight got 2 shillings a day and an ordinary man-at-arms half that, a shilling. In July 1355 the crown's rates for its army in Gascony kept the same shilling, against 3 pence for a foot archer, so he earned four times as much. Out of his pay he had to keep a warhorse, one of his biggest costs. In England the horse usually had to be worth at least 100 shillings, or £5, which was a hundred days of his pay. Before a campaign it was valued and its looks written down, so that he could be paid back if it died or was killed in battle. An ordinary squire's horse might be worth only £5, while a great nobleman's could be worth up to £100, twenty times as much. From the battle of Dupplin Moor in 1332, English men-at-arms often fought on foot beside the archers.
Sources: en.wikipedia.org · themcs.org
What a man-at-arms's pay bought in 1350
a day of a master mason: less than a day's pay
a fur-lined garment: a week's pay
a hand-copied book: 3 weeks' pay
a great stained-glass window: 3 years' pay, more than a whole year of it
a warhorse: 4 years' pay, more than a whole year of it
Every price of 1350, with its source, is on the prices page.