Private insurers do have a profit motive, but with the right regulations, that can help, rather than the US system where it encourages more spending. There are plenty of countries with universal coverage that rely on private insurers - the difference is the system is set up to control costs while providing a pretty standard set of benefits and cost-sharing.

So one reason why Medicare Part D works pretty well is because the private insurers do the negotiating on drug prices. I've read reports that they actually get better prices than for their private plans (there is a ton of competition for Medicare prescriptions).

Are the prices lower than Canada? No, not generally. But they are some of the lowest prices in the US (excluding Medicaid which has a mandatory discount).