Considering the Rockbreaker Ox encounter - this suggestion is to let us choose the bar type we give it, for example, as it is 5 silver bars in the text, allow us to choose either 30 copper bars, 15 iron bars, or 5 silver bars. That way there is a sink for the large amounts of bars we can get (I myself have a Lot of iron) but the value of the rarer bars still says higher than more common types.