the estimated cost of M4A is so high that your income and Medicare taxes would have to be doubled -- or, you could pay an estimated eighteen  percent Value Added Tax on everything you buy -- including rents, leases, auto insurance, food, clothing, implied rental value of owned housing and automobile, auto lease, all repairs, etc.
The moment you mention this to previous M4A supporters, the majority turn against the idea. [Which, of course, is why Bernie doesn't discuss the cost or required taxes.}
Buy In would, in essence, replace ACA. It would deplete the Medicare trust funds at a significantly faster rate [there are 20 million on ACA and about 50 million on Medicare but the average actual medical cost of ACA is far higher than Medicare, so the Buy In program would run at a significant annual loss. My prediction is that Medicare taxes would have to be increased 50 percent to support this. That 1.45 percent of wages tax increase is likely invisible to the typical worker [it'll be entirely covered by not getting a raise in 2020] and so I expect that would be the people's choice of how to proceed.
If, of course, anyone would tell truth and let us have our say in honest fashion. Frankly, I doubt that'll happen.
Added comment: Buy In might be cheaper than the cost of some current business health insurance offerings and so there might well be more people using it than estimated by ACA enrollment figures. Which might require raising the required taxes even more ... but would still seem cheaper to most and would preserve the "Cadillac" plans of unions and big business [which they will fight to protect]