Depends on the location. Using cigarettes as an analogue... they are really cheap to manufacture, but some places like NYC tax at a rate even greater than 100% (nearly $6/pack flat tax last I checked, which means once you add in margin and vendor costs you are looking at $10+/pack which is brutal for an impoverished pack-a-day addict). So as a result there is a massive bootleg market where cigarettes are smuggled in from low or no-tax areas and sold illicitly.

I'm not super familiar with street prices of drugs, but if politicians tax drugs the way they tax cigarettes and alcohol, it will be burdensome for drug addicts and cause increased theft and bootleg operations that have some (but not all) of the same problems as drug trafficking.