Based on this 14th c. record of contributions to ransoms, a decent rule of thumb for a gentleman’s ransom might be one to two year’s income. In that record the contribution to the knight’s ransom was less than that, but it seems likely that, unlike the yeomen and squires, he had sufficient means that he was expected to pay part of the ransom himself. Lord Bourchier's ransom agreed to in 1374, equal to about 1300 pounds sterling not counting his expenses in captivity, seems to have amounted to a bit less than two years of income from his lands.
An ordinary squire, then, might expect to pay 18-36 pounds sterling, a simple knight twice that. At 240 silver pennies or sixty silver groats to the pound, that’s an inconveniently large number of silver coins. In late 14th c. England a gold noble was worth a third of a pound and weighed a third of an ounce. 18 pounds sterling would be 54 gold nobles weighing 18 ounces in total. That is also an inconveniently large number of coins for a modern recreator of the middle ages.
A ring, brooch or other piece of jewelry set with a gemstone could cost about that much, depending on the value of the stone. A number of consensual deeds of arms in the 14th and 15th c. specified that the loser could ransom himself with a gem or piece of jewelry. For the participants, not specifying a specific monetary ransom had the additional advantage of making the enterprise feel less crassly commercial. For modern recreators of medieval deeds of arms, reproductions of medieval jewelry are a practical alternative to a heavy purse full of reproductions of medieval coins.