What would the ones we know about be worth in modern value?

It's very tricky to the point of impossibility to convert from ancient value to modern value. Do you just try to directly convert the value of a precious metal? Do you relate it to the GDP of a country over time? Do you use the face value of a pound instead of the value of the raw silver? Do you tie it to the average person's wages? They're all valid approaches, but they will tend to give very different answers.

The most famous would be the ransom of the Inca emperor Atahualpa, who was captured by Francisco Pizarro in 1532. His ransom was 13,000 pounds of gold, and 26,000 pounds of silver. At today's rates ($17,461 per pound of gold and $230 per pound of silver) this works out to about $227M in gold and $6M in silver. Atahualpa was executed anyway by Pizarro.

King Richard I's ransom in 1194 was for roughly 60,000 pounds of silver, which is roughly $14M at today's rates. If you consider instead that it was two to three times the entire annual income of England, then you can argue for a substantially higher worth, proportionally.

otherwise interesting ransoms...

Keep in mind that these were far from normal cases. Rémy Ambühl's excellent dissertation on ransoms in the Hundred Years' War states that the average ransom (for a commoner) was a mere six to seven pounds. (The most lucrative ransom of a commoner reported in the dissertation is for 66 pounds.) It is, however, full of interesting stories regarding the ransoming process. What happens if your prisoner turns out to be a brigand instead of a French soldier? (Answer: execute him, and then nicely ask the English courts to reimburse you for feeding and quartering him for five weeks.)

Were prisoners sold as well? Absolutely. Joan of Arc was sold for 1,666 pounds.

Could you buy a prisoner and try to sell him or her at a profit? Indeed, yes, as well. A man named Waleran de Luxembourg was captured in 1374, and sold to John, lord of Gommegnies for 1,667 pounds. John "flipped" the prisoner by selling him to Edward III for 3,334 pounds. While Edward acquired the prisoner in order to trade for the release of one of his knights being held by Charles V, the French king refused to trade, and so Edward III ransomed the prisoner for 16,667 pounds, a very lucrative 500% profit.

When John II of France was captured at Poitiers his ransom was three million gold écus (crowns) and most of southwestern France (treaty of Brétigny link). This wasn't even the worst version, the treaty of London (which wasn't agreed to) would have had the ransom at four million écus and would have meant the loss of almost half of france (everything west of the black dotted line in this map), while the English king could have kept his claim to the French throne. Of course this isn't quite as clear-cut as Richard I's case, as the French had just lost a war and loads of important nobles, but the cash part of the treaty seems to have been a ransom.

I tried to find a modern equivalent for the ransom, or the relationship to the annual income of France, but I couldn't seem to find it. It might be difficult to work out as John had to devalue the currency in an attempt to pay the ransom. I'll try to get more sources for this soon.