Couple of problems with this.
1) the vast majority of drug users are recreational drug users, if it got to the point they couldn't afford them they would not turn to crime, they would simply not do it, I know there is a percentage that are addicts and would turn to crime, but they are the minority that always gets reported in the papers (people going out, taking drugs, not getting into trouble or causing a crime wave and then going back to a normal job after the weekend doesn't make good headlines)
2) if they were legal they would be taxed to the level that it would probably still be cheaper to get them from the old dealers through the existing supply channels rather than from a chemist