If surveillance has become more common in recent decades, it might be because it's so much cheaper. A new report from security researcher Ashkan Soltani and Kevin Bankston looks at the hourly cost of the surveillance techniques used in various Supreme Court cases, and emerged with some interesting figures. The paper was inspired by the common question of how GPS tracking is any different from simply following a suspect in an unmarked car. In the new report, the answer is more than $200 per hour.

The report looks at four techniques — covert surveillance, beeper tracking, GPS tracking and cellphone tracking. The cellphone is by far the cheapest option at $5.21 per hour, with GPS clocking in at $10 an hour and an old-fashioned tail costing the authorities a whopping $275 per hour. The point, according to Soltani is that "privacy protections are often made possible due to structural transaction costs." As the cost or surveillance drops, it becomes used more frequently, a particularly sobering thought in the context of low-marginal-cost activities like bulk email collection. "Once the cost approaches zero," Soltani writes, "we will be left with only outdated laws as the limiting function."