Drug tests aren't that expensive.
They have instant ones - they're like 5 bucks.
That's the cost of the test. The test also has to be administered. If you're going to trust drug users to administer their own tests, you may as well not give them one, genius! You can even buy clean piss in some
gas stations these days, for crying out loud, and its readily available in head shops or online. The only way to administer a drug test and be sure the urine is from the test subject is to observe the piss come out of the test subject and into the sample vial, and that my friend will cost more than $5 a pop.
As someone who has paid taxes all his adult life, god forbid should I ever need public assistance, I would find no greater insult than to be required to allow a government employee to observe me pissing in a jar.