Well, to play devil's advocate, I'll state that the US is larger than most countries with single-payer. Also, as Hamradio mentioned, without some kind of adjustment to medical costs, the price tag would be astronomical . And finally, as many studies have shown over the past decade, Americans are generally in poorer health, not just due to lack of healthcare, but lack of exercise, dietary habits, etc.
Single payer would be enormously expensive, and it would require a real shift in the way things are done in the US The defense budget would have to go way down, taxation would have to go up a lot for many people, and there would most likely be an overhaul of the entire medical field, from top to bottom. I don't know when or if most Americans would be willing for any of that to happen, let alone the powerful lobbyists.
I searched a little, but could not come up with a good reference for how much it would actually cost for a single-payer system in US.
Below is a Washington Post editorial in which they cite one estimate that it would cost $32 Trillion over 10 years. While that may be true, what about offsets with revenue from people covered, physicians, hospitals, employers, etc.?
One aspect is that the federal government already provides substantial medical coverage for employees, military and dependents, military retirees and dependents, VA, USPHS, and many others. Not to mention that provided by state and local governments and school systems.
I still believe the answer is a truly bi-partisan commission to do an 18-24 month study with all stakeholders involved.