For most americans, it should be less than what they pay for their healthcare right now.
You can also do it the public option route that allows people to choose rather than be forced into that system. I think it's an inferior solution to a true single payer but it might be easier to pass.
No, what prevents the US from having a single payer program is that the health insurance industry knows it will be pretty damn close to a death sentence for them (yeah, there will be a room for supplemental insurance, but that's a much smaller market than now) and they would fight tooth and nail to kill it.
They did it in the 90s and that's why democrats went with an approach that wouldn't piss them too much.
Oh, it would absolutely be superior and less expensive for us, but convincing people of that is the challenge. Too many people don't learn shit about economics, personal finance, and even struggle with basic arithmetic. It'd be really difficult to overcome the massive GOP and insurance industry media blitz that would happen.
I think going the public option route is the best shot at easing into a more socialized system. As Bernie's popularity has risen quite a bit, I think public option might be able to make it through at some point as a good "compromise" between the current Obamacare status quo and Bernie's medicare for all stance. Over the long term, I think liberal Democrats and Independents need a bigger PR effort to sell more people on true universal healthcare, showing the benefits, while also refining existing proposals.
I feel like too many Democrats tend to be too reactionary and short term thinkers. Conservative/Neo-Conservative policy is a complete disaster in every way imaginable, but they played the long game with that, building towards the current situation over a period of decades.