A price tag was a small sign affixed to an object that listed the amount of money an item cost.
The term could refer, metaphorically, to the collateral cost of one's rash or otherwise unexplained actions. In 2268, Leonard McCoy criticized James T. Kirk having made an obsession-fueled decision to vanquish the dikironium cloud creature once and for all, after two crewmen were attacked by the creature as it entered the vent of impulse engine number two, leaving one dead and the other with a slim chance. McCoy told Kirk, "You can add that little price tag to your monster hunt." (TOS: "Obsession")
It could also refer to a favor that somebody wanted before they would do a favor for another. In 2375, a skeptical Kathryn Janeway surmised (correctly) that gaining the assistance of the Think Tank in avoiding the Hazari came with a price tag. (VOY: "Think Tank")