The problem is when it's totally socialized (like medicare), you end up with people abusing the service. If an ambulance ride costs you less than a taxi or even bus, and you're an asshole, you use an ambulance (in some places, provided by paramedics, so the direct costs are high -- in some places they also send a fire truck, so you end up with like 20 highly-trained guys and $500k of equipment) to go to a routine checkup. 3-5 times a week.
You could probably waive charging direct costs if you had some other way of deterring abuse, but they essentially always have to respond to 911 calls, so the only way I could think of doing so would be to arrest someone who misuses the service. A policy of informally losing the bill for legitimate calls might work, or doing the standard drug dealer "first one's free" strategy. But I don't think $100-500 for an ambulance would break most people, particularly with insurance; a charity could also cover the bills for some people.
You could probably waive charging direct costs if you had some other way of deterring abuse, but they essentially always have to respond to 911 calls, so the only way I could think of doing so would be to arrest someone who misuses the service. A policy of informally losing the bill for legitimate calls might work, or doing the standard drug dealer "first one's free" strategy. But I don't think $100-500 for an ambulance would break most people, particularly with insurance; a charity could also cover the bills for some people.