I wrote a longish post about this in TIL a bit back, abut the value of the seventy five Attic Drachmas Caesar willed to every Roman citizen. To cut a long story short, a Roman soldier was given 225 denarii a year, a US solider is given a base pay of about $20,000 a year, so probably the best way to think of the value of a denarius as about $90. This fits somewhat nicely with other wage data we have, which often shows unskilled labor being paid about half a denarii a day. So a denarius is about one to two day's worth of unskilled labor.

It is also sixteen glasses of cheap wine or four glasses of fancy wine at Hedone's Bar in Pompeii.