Costs us a lot more than a coffin. Raising a child is an investment by society, and to have that kid bump himself off at 20 is a waste.
Regardless, reckless drug use is associated with societal problems, as least as far as economic production by individuals is concerned. Although marijuana may be controllable, things like meth and heroin DO fuck you up, and you just cannot be as productive as you could be without these drugs.
An argument in favor of government banning illicit drugs, (and I think it is a good one) is that the majority of society does not use these, and the consequences of using these hard drugs almost inevitably puts a burden on society in the form of shelters, welfare, etc. Thus, diligent, hard working individuals may, in fact, have a right to demand the government ban clearly harmful substances (again, marijuana doesn't quite fit this description IMO), because they have invested in public education and social welfare schemes, so they have a point to remove young people's reckless destruction of their bodies.
A counter argument may claim that there is no direct harm to these diligent people, so they should butt out. But if we are to stay ahead of superior Japan and huge motherfucking China, is it not entirely out of reason to ban substances?