That could be arbitrary based on the various assumptions. I am attaching a discussion and a link from Answers.com that may explain this in more detail. It looks as though at the time it was issued, its purchasing power was worth about $20 or $21 in 2005 dollars. The silver value is worth about $1.50 to $2.00.

From Answers.com :
It is problematic to give even rough comparative values for money from before the 20th century, due to vastly different types of products and of the impossibility of making an accurate price index based on vastly different spending proportions. Its purchasing power in terms of bread has been estimated at US$21, from 2005, in the first century. Classical historians regularly say that in the late Roman Republic and early Roman Empire the daily wage for an unskilled laborer and common soldier was 1 denarius without tax, or about US$20 in bread.[citation needed] (By comparison, a laborer earning the minimum wage in the United States makes US$58 for an 8-hour day, before taxes.) The actual silver content of the Denarius was about 50 grains, or 1⁄10 troy ounce under the Empire. In 2010, this corresponds to approximately US$1.70 in value if the silver were 0.999 pure (which it wasn’t).

http://www.answers.com/topic/denarius