The excellent Geoffrey Chaucer Website has an interesting record of payments made to ransom young Geoffrey Chaucer, then a valettus or yeoman, and various others.
The king paid to ransom not only one of his squires and various valetti (£8-16), but also humbler sorts like eight carters (£12 total), two poultry purveyors (£10 the pair), and an archer (40s)
These ransoms seem to have ranged between three years and half a years income for the individual ransom. The king paid a higher ransom for one of his squires than for a knight, perhaps because the knight was expected to pay part of the ransom himself.
There's a lot of good material on the site: other contemporary authors, Chaucer's sources, life and manners, science and courtly love.