What did a medieval royal mistress earn?
2,000–3,000 pounds a year in land and gifts, by Parliament's count.
About $610,000 a month in today's gold.
A month of that pay weighs about as much as a five-litre bottle of water in gold.
Fifty-six manors in twenty-five counties, a fortune in jewels, and one very old king.
Where the money came from
Alice Perrers entered Queen Philippa's service as a lady of her chamber before October 1366, and her bond with Edward III may date from that year, when she was granted two casks of wine. Her income came as royal favours, not wages. In 1367 she was given the keeping of Robert de Tiliol, with his lands and the right to arrange his marriage. In 1371 the king gave her the manor of Wendover, and on 15 April 1372 £397 was paid for her jewels. She also lent money: in 1367 a baron borrowed from her to join a crusade against the pagans of Lithuania. In 1373 men probably acting for her bought the manor of Pallenswick at Fulham, and before her marriage she could afford Egremont Castle. At her height she held fifty-six manors, castles and town houses in more than twenty-five counties, and only fifteen of them were gifts from the king. In 1375 she rode from the Tower of London dressed as the Lady of the Sun, to a great joust. In 1376 Parliament sentenced her to banishment and the loss of her lands, and in 1379 the treasurer delivered 21,868 of her pearls to the royal wardrobe. The £397 paid for her jewels in 1372 equalled a haymaker's pay for more than 95,000 days, at the penny a day that the law of 1351 allowed.
Sources: en.wikisource.org · british-history.ac.uk · en.wikipedia.org · en.wikipedia.org 2
What a royal mistress's pay bought in 1350
a year's food for one adult: less than a day's pay
a knight's full armour: 2 days' pay
a warhorse: 2 weeks' pay
Harlech Castle: 3 years' pay, more than a whole year of it
the ransom of the King of France: 200 years' pay, more than a whole year of it
Every price of 1350, with its source, is on the prices page.