On terra, this was quite common and important hostages would be kept for lengthy periods... up to decades... waiting for the negotiations to conclude, People being people (quite gauche and immune to the effects of shame and bad taste when it comes to money, and additionally with the existence of gods of trade who promote bargaining) I would assume it would have to be the same in Gloranthan lands that have ransoms.
I can understand why one would want to hold to higher ideals and views of people, especially in a FRP, so I would say if one wished it to be so, simply make it a binding taboo backed by religious decrees and societal relations and have your game be set in a better light than I suggest. Personally, I fear that money sullies noble thoughts or activities when it comes down to it in the end.