That's great. It would be good to know just what would be excluded.
For me, then, if I look at this, let's say we have a single mom, unemployed, who takes her kid to the doctor's office and doesn't have benefits. Under this model, she would get a prescription from her family doctor or nurse practitioner. That prescription would be tested against a formulary of some kind—hopefully one that's national and put together by the feds, the provinces, and the territories—and she would then go to a local pharmacy and have the prescription filled.
Depending on the means test, there may or may not be a $5 copayment; I guess that's whether or not she chooses generics. The cost of those meds, if we went to a single-payer system, would likely be covered by the province or territory through the existing payment systems in place in pharmacies for the ODB and other provincial plans. She would be able to go home.
The cost to us of the government doing that would be about $20.4 billion, and we already have more than $23 billion being spent just on employed people in Canada, so we could set that mom up with an insured plan and still save public funds on top of the $4 billion that you've identified as savings.