So the purchasing power of a state entity - exactly the solution I proposed for the US. The state has subtle levers of control over all the participants in this industry, which other players (no matter how large) simply don't have.
This is not an objection to using single-payer in the US, quite the contrary.
The real answer as to why Canada gets lower prices is gov't fiat - they just say "this is what we will pay you, take it or leave it".