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.
When Scotland united with England in 1707, the value of the Pound Scots compared to the Pound Sterling was 1/12, making it essentially worthless to refer to something as being worth only a Scots halfpenny.