The more interesting hypothetical is: would a NICU stay still cost $1M if the factors inflating that cost (namely, insurance subsidizing hospital revenue) were no longer in place?
I can assure you it wouldn't be $0, or anywhere near close to that.
My kiddos were born the day before the third trimester and spent two months in the hospital. Staff it at minimum wage, cap the costs at just the stuff used, and you're still talking a lot of money.
No one is making the claim that the cost of service would be $0. Quite obviously.
What you can't be sure of is the real cost in terms of resources, and how that would be accounted and paid for. The price of your childrens' stay, to you personally, could be far less than what you originally paid personally if healthcare was not reliant on private subsidy. You would also likely be subsidizing the costs of other children staying in the hospital through public subsidy using your tax dollars. I reckon that last point is where a lot of people in the US get hung up.
Which charities have the funding to cover a $1M NICU stay?