The guy doing the testing at our field gave come crazy figure of 1 in 1000 positive drug detections and those tongue scrapers cost $40 each.
Do the numbers from there !
Let's say he can do those in a couple of months, include his salary and car etc and you'll see it costs a lot of money to catch a pilot or crew member using drugs, but it's hard to put a cost on a life or more should the worst happen...