In just five lines, the poem lays out a cold bargain. War, trade, and the hangman are grouped together as if they were the three parts of a single "commonwealth," a word that normally suggests shared wellbeing. Here it sounds ironic, because what these three share is a way of making money. Violence, commerce, and punishment all work together, and somebody profits.
The final image is the strangest and most interesting. A "cheap piece of star" could be something small and beautiful, like a bit of light or a dream, or even a real object, such as a star-shaped medal or a coin. The poem doesn't say. Either way, something that seems precious gets turned into a bargain-priced commodity. The title, "Mercantilism," points to an old economic idea in which nations compete for wealth, and the poem seems to suggest that this competition feeds on suffering.
The speaker's tone is flat and detached, which makes the poem feel bleak and quietly accusing. No one is shouting. The calm delivery is what chills.