Should proably read your own sources...

Drug testing is expensive. The average cost of a drug test is about $42 per person tested,[8] not including the costs of hiring personnel to administer the tests, to ensure confidentiality of results and to run confirmatory tests to guard against false positives resulting from passive drug exposure, cross-identification with legal, prescription drugs such as codeine and legal substances such as poppy seeds. Another way to measure the cost is by counting what it costs to “catch” each drug user. Drug testing is not used by many private employers because of the exorbitant cost of catching each person who tests positive. One electronics manufacturer, for example, estimated that the cost of finding each person who tested positive was $20,000, since after testing 10,000 employees, only 49 tested positive. A congressional committee also estimated that the cost of each positive drug test of government employees was $77,000, because the positive rate was only 0.5%.[9]

There's more to it than the cost of running each sample.