Hi,
In my previous employment we would often undertake D&A testing of subcontractors working on our sites as well as our own site management staff. This would involve an external specialist coming to site (PM me if you want their details) who would select 10-12 people at random from the signing in book.
Costs, if i recall correctly, were in the region of £50 per person. However, if somebody provides a "non-negative" sample on site (you don't use the word "positive" at this stage) then there would be additional "chain of custody" costs as the sample is sent off to be analysed.
In the interim period (usually 3-4 day) the person was exlcuded from site. If the sample came back negative then they would be allowed back onto site. If it came back positive they were banned from all company sites for 3 months and would then only be allowed back on if they could provide a clean bill of health. Direct staff, if found to have provided a posituive sample, would be dealt with at company level under the company's internal disciplinary policy.
The testing kits they use are, we're told, 99% accurate (hence the need to send off any non-negs to the lab for a 100% answer). However, it should be noted that in 8 years of testing, every "non-negative" sample we ever had on site came back as positive.
Testing method was urine for drugs and breath for alcohol. I would however thnk carefully about how you choose to test for cannabis. The reason i say this is that cannabis can be picked up for weeks, sometimes months, in a urine test and every test would result in at least one or more people being removed from the job for cannabis.
Hand on heart i can say that none of them were ever "stoned" on site and usually just smoked at weekends, parties etc (the Amsterdam stag do was a big one as well!) Unfortuanately a urine test won't give much indication as to when they smoked it, meaning a lot of good guys ended up losing work for a few drags on a joint.
At the time i pushed for saliva tests, which we were told had a "range" of around 24-48hrs, rather than several weeks. This way anybody who had smoked it recently would still be picked up, but those who had smoked at a party, stag do the week before wouldn't.
There was of course an additional cost per person for the saliva tests. However, when this was weighed up against the reduced number of non-negative samples and associaited "chain of custody" costs, the overall increase was negligible.
Hope this helps!