The person you replied to specifically said prison is half the cost of alternative services to drug addicts. Your refutation would be worth something if you had figures, as they have subsequently posted a link to back up their position.
In fact, the author encourages services to drug addicts near the end:
> With a secure emergency shelter system like that in San Diego, the county and city governments can reroute existing resources and “flood the zone” with on-site treatment options for the homeless. For addiction services, we should prioritize recovery programs and terminate policies like safe-injection sites that draw addicts from other cities.