The purchasing power of gold has not diminished since Biblical times. According to the Old Testament, during the reign of King Nebuchadnezzar, an ounce of gold bought 350 loaves of bread. Today, an ounce of gold still buys 350 loaves.And of course it is a myth, one of those urban legends that has been debugged to death but still won't lay down.
However, as a Bible scholar, Claude Mariottini is not afraid of the truth:
To say, however, that one ounce of gold in the days of Nebuchadnezzar bought 350 loaves of bread, one must assume several things. First, one must assume that the ounce, a unit of weight in the avoirdupois system, once used in the United Kingdom and still used in the U.S. system of weights, was also used in Babylon. Since the Babylonians did not use imperial units, this statement is false.Read all about it here.
Second, we must assume that the value of gold has remained stable in its relative value to the price of bread. If this assumption is correct, then we must also assume that the price of gold and the price of bread has remained relatively the same for the past 2,600 years. It is evident that no one can assume that this is true, therefore, the statement above also cannot be proved.