The direct cost of trying to enforce this prohibition on drugs is enormous.
http://www.drugsense.org/cms/wodclock
http://www.forbes.com/sites/artcarden/2012/04/19/lets-be-blunt-its-time-to-end-the-drug-war/
The cost in lives lost is commensurately large - 60,000 estimated dead in the war on drugs in Mexico.
Drugs revenue is also enormous - all of which, since it is illegal, represents untaxed income.Estimates vary, but it must represent billions lost to the revenue services around the globe.
Drugs should be legalised and commercialised in the same way as for alcohol, and with the same restrictions, including age restrictions, penalties for being under the influence whilst driving, operating machinery etc.
Should free up some police time,there should be a reduction in crime, and it would increase revenue to the exchequer. There are still drugs you would probably wish to prohibit - crystal meth, heroin etc, but most of the "recreational" drugs ought to be legalisted, in my view.