You can ransom common prisoners (soldiers, bandits, etc.) for 50-300 denars. Kingdoms will often offer to ransom any captured lords in your possession for anywhere between 1,500-5,000 denars, while leaders (kings, queens, etc.) go for much higher prices (even 80,000 denars at times).
As selling prisoners to Ransom Brokers can be a highly profitable enterprise, especially early to mid game, there are a number of ways to increase the number of captured prisoners. One way to do so is simply to order your troops to use blunt weapons to knock the enemy out. Another way is to recruit Manhunters to your army, since they exclusively use bludgeoning weapons and can be upgraded to potent light cavalry, though this can be difficult as the only way to acquire them is to recruit them from prisoners. Finally, one shouldn't ignore the possibility of equipping a bludgeoning weapon and hunting down prize enemies personally.
Common prisoners can be handed over in taverns to wandering Ransom Brokers, however, lords can not be ransomed this way. You have two other options with lords: you can hold onto them and wait for their kingdom to offer a reward, or sometimes a competing kingdom will offer to take them off your hands when talking to their lords or leader. If you don't hand them over to someone else, you will most likely receive repeated events over time requesting that the captured noble be returned to their native kingdom for a price. If the offer is rejected, the player's reputation with the nation in question will decrease, as will their honor. Oftentimes, the request will be repeated, occasionally with a higher price.
It may be in your interest to keep lords as prisoners for as long as possible, an opposing nation can only have as many armies as they have lords. So if the player captures 7 out of 40 lords, only 33 armies could possibly be in play on the opposing side at any one time.
In the Nordic city of Tihr there is almost always an NPC named Ramun the Slave Trader who resides there and acts as a Ransom Broker, but only pays 50 denars per captive, regardless of rank. As only the bottom tier of soldiers and bandits can be ransomed for a lower price than this, it is generally considered a waste of money and prisoners. Ramun can be found in With Fire & Sword too, but the ransom cost, although it changes from soldier to soldier, doesn't change in practice (13 thalers per soldier sold). Furthermore, the theoretical price he gives isn't analogous to the soldier's experience (for example, a Rebel costs 160 thalers while an Armored Cossack costs 51 thalers).
No nobles may be sold as slaves.