As a general rule, most cultures observe the custom of taking prisoners for ransom.
In areas ruled by a lord, even common folk may be ransomed (although they are paid for and released as a group, and their price is far lower than that of an aristocrat). If their lord fails to pay, the custom is that the prisoners become serfs of their captor. In this circumstance their previous lord loses much status, and it is this, even more than the value of their labor, that is the main incentive to pay.
Lords will generally not make prisoners of free peasants, as they are rebellious and unreliable, and unlikely to have the resources to pay a ransom. They will instead kill them.
Free peasants, by contrast, will generally take prisoners of serfs. They may let them join the community after a certain period. Otherwise the custom is to brand or tattoo them with a distinctive mark and let them go. Anyone with such a mark taking arms against them will be killed on sight.
Free peasants generally have a great hatred for aristocrats, and so may kill them on sight. However they may capture them due to their high ransom value. The disgrace of a lord being out-fought by, and surrendering to, peasants, can work both ways. Sometimes the aristocrats will pay an unusually large ransom very quickly. At other times they will deny that it happened at all, and hope that the peasants kill the lord.
Inanimate objects, such as important religious relics, may also be taken for ransom.
Outside of big cities, prisoners are generally expected to work as if they were members of the community.
Note that, since barter is more common than currency, most of the time ransoms will be paid in kind, for example with livestock.