We currently spend $3.6 trillion on healthcare so I think that $3.2 trillion is just a rough guesstimate going off of that.
We spend $1.2 trillion on health insurance premiums. $335 billion on prescription drugs. $375 billion on "out of pocket expenditures". I don't know what pharmaceutical companies and insurance companies profit margins are, but if we eliminate OOP expenses, and take away 20% from the insurance premiums and drugs to account for the profit, that could cut healthcare spending by $682 billion right there which puts you to $2.9 trillion.
And then you get into the more abstract benefits like people will start going to the doctor for routine check ups instead of waiting until they have a catastrophic medical emergency that will end up costing way more money.
$1.9 trillion for a well managed single payer healthcare system in the U.S. could be possible.