That default rule is terrible.
Its an old holdover from AD&D.
EDIT: strike that, DnD actually said 10 coins weigh 1 lb. ABSURD
Coins have very rarely ever weighed that much, historically.
Just think about how much 100 coppers will buy you, yet you have 10lbs of copper? Utilizing the DnD method.
Even 160 coppers still aint crap, though.
An ounce is roughly 27 grams, most large coins rarely weigh more than five -seven grams, smaller (normal coins weigh usually 2-4 grams.
Find a metric scale and put a penny on it or a centime or a euro-penny whatever.
A great and well researched article can be found in this PDF, which coincidently is all about the insanity of coins weighing an ounce/ per coin.
See http://www.dragonsfoot.org/php4/archive.php?sectioninit=FT&fileid=261
the article is within the e-magazine.
EDIT: Further note the conversion ratio of silver pennies to gold lunas would be astronomical were historical exchange rates used.