I am not certain that the cost of incarceration is higher than the cost of the property theft and damage by drug addicts. Cost of incarceration seems to vary between 15 and 40 k$/yr in countries like Canada, USA, and England. Property theft and damage costs vary widely depending on the individual addict, and can be between 0 and 1000000 $/yr. In my city, there are many drug addict thieves who have been convicted of hundreds of car thefts each, and that is probably only a small fraction of the number they have committed. It is also common for these criminals to break a car window (costing hundreds of dollars) to get a few dollars that were sitting in the car; this can add up to a lot of damage very quickly if one needs to make as little as 50 $/day to feed an addiction. Another common income source for these people is to steal items like computers, cameras, and bicycles, which they sell for as little as 5% of their purchase cost; this can cost 1000 $/day for a 50 $/day habit, adding up to >350 k$/yr (roughly 10 times the cost of incarceration).

I should add that I am in favor of full legalization of all substances (including the elimination of the requirement for a prescription to obtain pharmaceutical drugs), but I do not pretend that this will reduce the harms which inhere to drug users, or the rest of the citizenry.