Just some facts to consider. I got many of them from:
https://www.kff.org
At first I didn't trust that link, because it's the family foundation of a health care empire, but I cross-checked some of the facts with US gov sources, and they appeared to be quite accurate. I liked the KFF site better only because the information is much better organized.
So... it would appear that the Medicare and Medicaid programs currently cover 135 million people at a cost of ~$1.3T. (It does seem weird to round to the nearest $100 billion dollars, but whatever.)
The Veterans Administration provides benefits to another 9 million people:
https://www.va.gov/vetdata/docs/Quickfacts/Homepage_slideshow_06_04_16.pdf
https://www.va.gov/budget/products.asp
At a cost of $200B.
So there are currently about 144 million US citizens covered by government healthcare at a cost of about $1.5T. The current estimated population of US citizens is 328 million, so the covered group is 44% of the population. (Some people get supplemental coverage for Medicare, but that's only about a $30B industry, and I'm including rounding errors greater than that, so I'm ignoring it.) I figure the currently covered fraction of the population is probably unhealthier than US citizens on average, due to the nature of the before-listed programs, so the incremental cost of covering everyone will probably be lower per capita.
The private health insurance industry had revenues of about $840B in 2017, covering 176 million people, making the private companies look more efficient than the government, but I suspect a lot of this coverage is inadequate. So let's fudge the whole thing and say that covering everyone in the US under Medicare and Medicaid would just double government healthcare spending to $3T, with a budget gap of about $800B, assuming you confiscated all private premiums and put the private insurance companies out of business in this sector. Let's also assume that the insurance companies employees who were unemployed would find jobs with the USG and states, so the net unemployment would be irrelevant in this discussion as a budget factor.
$800B is significantly bigger than the total US defense budget, and I probably underestimated everything, especially first-year costs.
Medicare for All looks like it needs a massive overall national spending increase at the Federal level. But I'm sure the Europeans and the Canadians will be in full support of it! ;-)