Just to follow up on the coins debate:
It may make more sense to think of coined metal as being the reference and raw
metal as being discounted in value because of doubts about its purity and
difficulty in bartering it in everyday commerce. If you have raw metal in the
form of bullion with true weights and guaranteed assay, then it probably trades
at the same value as coin, but it is only suitable for large deals.
Maybe if you are dealing with raw metal you should assume that the owner of the
metal starts by assuming it has a value equal to the same weight in coin while
the other party starts by accepting the metal at 1/2 of the equivalent weight in
coin but the final value has to be settled using Bargaining skills.