It would cost whatever citizens collectively are willing to shave off of wealth and productivity with taxes. You wouldn’t set a monthly amount and then throw your hands up and say it’s unaffordable. You’d start with the max amount you could reasonably convince a majority of citizens (who vote) to support and work backwards.

I’m not an expert, by any means, but to say it’s impossible seems flippant and intellectually dishonest. A better way to approach it would be with caution and the understanding that a lot of research and effort is going to be required if it’s to be implemented soundly (or determined to be untenable).