They actually cost a little less, overall. But only because non-smokers, on average, live about 10 years longer.
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But, in that case, you would have to take into account that most of the medical costs for those non-smokers are going to be near the end of life, most likely after retirement age, or at least after reaching Medicare age. So, as far as insurances that are offered as worker plans, within that time frame, the smokers are more likely to cost insurance companies more in health care costs, especially older smokers.
Basically, those insurance companies that are providing health care for workers are not interested in the overall costs of a lifetime smoker compared to a lifetime non-smoker, since the increase in cost of a non-smoker is likely to come after the person no longer has that particular insurance.