I know there are a lot of good responses to this argument ("you'd be paying for a jail cell anyway", "it's a net benefit to society", etc), but you have to remember that there is a very large portion of society that think drug users are inherently terrible people unworthy of help and won't listen to any logic beyond that. Although HN is made up of people who are willing to think through arguments fully and recognize why this program would result in a net-benefit to society (at least, in theory - it hasn't been tested yet), it's important to remember that people exist outside the HN bubble. To these people, the idea of helping someone obtain drugs (especially the government helping someone obtain drugs) is a terrible idea, hard stop.
It's certainly not the drug itself that is so expensive; the unit cost of an individual dose of any of the common opiates, when purchased through a pharmaceutical channel, is going to be less than a rounding error.
I know some people will still prefer punishing others to helping them, but at that point I don't think their opinion is worth much.