I'm aware of your and the GP's claim, I'm saying it doesn't survive contact with reality.

If you look at e.g. the per-dose price of insulin it's as low or lower in countries with single-payer universal systems, where someone requiring insulin is never going to have any idea what it even costs, because it's just something that's provided for them should they need it.

In that case it's usually some centralized state purchaser that has an incentive to bring prices down, or a government that has an overall incentive to keep the inflation of its budgetary items down, which ultimately comes down to public elections etc.

In any case, a much more indirect mechanism than someone who'd be directly affected paying the costs associated with the product, which directly contradicts this particular argument.