Sparks did a great job explaining that emergency services are ALWAYS marked up. Vet services, ER visits, ambulances, anything that happens on a moments notice costs more. Does it actually COST more to provide that service? No--but you're paying to insure that you can see a doctor immediately.
It costs more because my $300 insures that 911 isn't going to tell me "I'm sorry, the ambulance is all booked up until Monday, call back for an appointment then." A hospital is a private business, just like any other. The free market will generally regulate itself, and settle upon a fair price. If no one likes the $300 price tag, people won't use the ambulance as often (good or bad, depending why you're calling 911 in the first place...) and it will adjust.
It's not the same as bread. You can plan bread consumption. You don't need to be "on call" to make bread. You're grasping at straws with that analogy. My argument would still be the same though--if they charged $10 for bread, no one would buy it anymore. Economics says we'll look at substitutions--we'd buy bagels, crackers, or go without. Eventually bread would have to come down in price to a fair market value, where people are willing to pay for it.