What did a medieval earl earn?
About 1,500 pounds a year from land.
About $370,000 a month in today's gold.
A month of that pay weighs about as much as 3 litres of water in gold.
Fifty-odd families in the whole kingdom earned this much. One of them took in 11,000 pounds.

Where the money came from
Under Edward III there were never more than about a dozen earls and dukes at any one time. An earl lived on land. On 16 March 1337 Edward III made six earls in one day, among them his friend William Montagu, Earl of Salisbury, who was granted land and rents worth 1,000 marks a year to support the title. The grants of that year show that about £1,000 a year was the least an earl could live on, while £4,000 put a lord among the richest in England. War brought more. After 1341 great lords often signed contracts with the crown to bring a set number of soldiers, and the king's treasury promised to meet all their costs. In 1348 and 1353 the king promised the Earls of Warwick and Stafford 1,000 marks a year each, if each stood ready to serve with at least a hundred men-at-arms. The richest prize was a prisoner: Thomas Beauchamp, Earl of Warwick, took the Archbishop of Sens at Poitiers and ransomed him for £8,000. Under Thomas, Warwick Castle gained a new gatehouse and two towers, Caesar's Tower and Guy's Tower. In the army an earl drew 6 shillings and 8 pence a day, a knight 2 shillings and a pikeman 2 pence: the earl earned more than three times the knight and forty times the pikeman.
Sources: erenow.org · en.wikipedia.org · en.wikipedia.org 2 · en.wikipedia.org 3 · themcs.org
What an earl's pay bought in 1350
a year's food for one adult: less than a day's pay
a knight's full armour: 4 days' pay
a warhorse: 3 weeks' pay
Harlech Castle: 5 years' pay, more than a whole year of it
the ransom of the King of France: 330 years' pay, more than a whole year of it
Every price of 1350, with its source, is on the prices page.