We've had to use medical services out of province in Canada a couple of times (once in BC and once in Newfoundland at a hospital), all costs were directly covered using our Ontario health card, no additional costs. In terms of ambulance services I don't have any direct experience but I believe that ambulance costs are not covered in Ontario by the health card.
In the US I used a walk-in and another doctor a couple of times, I paid cash and got reimbursed by our insurance carrier, cost was $200 if covered by an insurance co., or $160 if you paid cash!! I guess that equates to the 80% coverage provided by the government plans down there.