I'm more specifically interested in how exactly they were used, I know ransoming rich and important prisoners was common, but for how much? What exactly would holding minor nobility get the captors vs someone more important like royalty or such?
How many important prisoners of war were ransomed compared to how many were used by the captors in other ways such as prisoner trades or political bargaining chips? Is there any specific examples of important POWs being used as political bargaining chips?
Was being killed in captivity, or simply dying due to poor conditions, common for political prisoners, or were they usually treated decently?
More can always be said (notably about the fate of low-value prisoners), but you may be interested in my answer about the captivity of Marshal Jean II Le Meingre, dit Boucicaut, a high-ranking prisoner captured at Agincourt.