How much would 'medicare extra for all' cost

My understanding is the uninsured are healthier on average, not less healthy. That is why they are uninsured, because a lot of them don’t think they need insurance.

America’s medical spending follows a power law distribution. 5% of people use up 50-60% of all medical spending, 20% of people use up 80% of medical spending. I would assume most of those people are already insured under medicare or medicaid.

So the uninsured should in theory be cheaper to cover.

The numbers vary depending on which data you use, but in general, the uninsured do not have health insurance primarily due to cost, not because they could theoretically afford the insurance but choose not to buy. (one sample dataset: Key Facts about the Uninsured Population | KFF). Unsurprisingly, the uninsured rate among the poor is much higher than of the “not poor” population (https://www.cdc.gov/nchs/data/nhis/earlyrelease/insur201802.pdf), and folks that live at-or-below 200% of the poverty line make up about half of all uninsured persons. This doesn’t necessarily mean that coverage costs would be higher for such persons, but lower incomes do tend to be correlated with poorer health statuses.

That $260 billion is an interesting number. It appears that’s enough to pay for almost all of the costs projected here to provide Medicare Extra to those who are currently uninsured.

This is the problem with these “half-assed” solutions. If an employer has to pay for an employee signed up on Medicare, then either (a) it’s less than the plan, stop offering a private plan, or b) it’s more than the plan, fire anyone who quits the plan.
… or (c) it’s set the same as whatever private plan is offered, so sign up for the cheapest, most useless plan and pay low dollars for all you employees to migrate to Medicare. By getting convoluted and trying to counter those gaming the system, it just makes for results that simply don’t work… like the incentive in ACA for employers to make personal coverage cheap but raise the cost of insuring additional family members, because the single coverage rate is the one that counts.

There are two types of uninsured - those who can’t afford coverage, and those who can but don’t bother. the former would have about the same costs as the general population, possibly more since their minor issues become aggravated through non-treatment. The latter are usually younger and healthier, pretty cheap to cover and bring down average costs. They are a reason why many group plans require everyone to participate.

Or a reason for funding the system, or the bulk of it, from taxes.

Yes, which is the ultimate “the whole group enrolls” concept.