Half that money is spent on enforcement, drug interdiction, overseas drug eradication programs, and local border control. The other half is spent on anti-drug advertising, drug education, and drug treatment programs.
Success is measured in a number of different areas -- interdiction is only one. Personally, I measure success by the percentage of the population that uses drugs. AND, I allow for the possibility that, under our form of government, we may only be able to reduce this number to a certain percentage and no further -- we may already be at that point.
So you'd consider a situation where half of the adult population smokes a joint a week, but doesn't drink or use any other recreational drugs to be a substantially worse situation than we have now?