A profit margin was a measurement of profitability, expressed as the percentage of revenue that constituted profit.
When Ferengi privateers captured the USS Enterprise in 2369, Commander William Riker told DaiMon Lurin that he hoped his profit margin was high enough to justify risking war with the Federation. Lurin assured him that it was. (TNG: "Rascals")
The Karemma frowned on greed, believing that all merchandise had a set value determined by the cost of raw materials, labor, and transportation, plus a reasonable profit margin. (DS9: "Starship Down")
During the Bajoran Time of Cleansing in 2372, Quark complained that the only thing being cleansed was his profit margin. (DS9: "Bar Association")
Later that year, Quark closed a vole belly deal on Ferenginar with a 15% profit margin. (DS9: "Body Parts")
During the Dominion occupation of Deep Space 9 in 2374, Quark told Kira Nerys that all he wanted was "peace, love, and understanding, not to mention a generous profit margin." (DS9: "A Time to Stand")
When Malon Controller Emck demonstrated that he had no intention of stopping his dumping of antimatter waste in the Void in 2375, Lieutenant B'Elanna Torres angrily remarked, "I guess mass murder doesn't factor into your profit margin." (VOY: "Night")
In 2381, Ensigns Brad Boimler and Sam Rutherford gave Kynk a presentation on how it would be 150% more profitable for him to convert his mugato poaching operation into a nature preserve. Kynk noted that it would be more work, but the profit margin was inarguable. (LD: "Mugato, Gumato")