The answer to your first question depends on what monitoring of the heroin users is put in place. The question that needs to be asked is will there be more users of heroin if its sale was legalised? The answer to that I would suggest is a resounding yes. The frailty and corruptabilty of human nature would guarantee that. Look at the failure of legalised prostitution in Victoria in stemming illegal prostitution. State legislation always carries with it a didactic component, thus in legalising things like the use of heroin the state puts the seal of legality and hence acceptability on the use of heroin.That gives these drugs a seal of approval for many young people who might otherwise be amenable to parental overtures.
It is not the impurities in illicit drugs that cause addiction but the addictive properties of the narcotic.
The real problem with narcotics (and other illicit drugs) is not only the deaths but the large number of users who, in various degrees, are unable to function according to their natural and educational abilities. Whether be within relationships or the broader community.Their lives become subservient to their addiction. That is the real cost.
Heroin does affect the central nervous system (but probably is not as serious in its effects, as methamphetamines and excessive use of marijuana is, on serotonin and dopamine levels in the brain and the destruction of neuron transmitters, which leads to chronic cycles of depression/mania, psychosis and schizophrenia) however it leads to massive damage to the vascular,pulmonary and repiratory systems. If there are impurities these can lead to infections in the brain and other body organs. It is strongly physically addictive which means it is a terrible addiction to beat.
Your second question is really a foolish one in light of the damaging effects of heroin. One young lady known to me ,since deceased, was spending up to $3000 per week on drugs including heroin and had to scrouge money for rent and food. With heroin it's a game of diminishing returns in terms of the user, so the cheaper the drug the more they will use.
