At the time of the political union of Scotland and England in 1707, the Pound Scots was worth 1/12 of the Pound Sterling, thus a "Scots halfpennyworth" implies worthlessness.
In historical context, when Scotland merged with England in 1707, the Pound Scots had a significantly lower value compared to the Pound Sterling, making it a worthless unit of currency to refer to something as being worth only a Scots halfpenny.