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And the long list of such trials are?

See @scottsanten's blog. He's posted articles about a lot of them.

Social Security is UBI for citizens over age 65. Yes it is failing.

It could easily be paid for by reducing your ridiculous military budget.

I would cut that too, to zero. Social Security and Medicare already take up 60% of the federal budget (military is about 20%) and they only serve mostly older people. How much will it cost with everyone enrolled? Likely several Trillion a year, an impossibly high cost we cannot bear.

Look at @scottsantens articles. He's costed it. It's not all that much. And the critical point is, actually, that we can't afford not to. Capitalism will collapse without a moneyed up consumer class.

I will.
So say in the US, how much should a monthly UBI payment be?

I read some of his articles, they seem to accidentally leave out the costs. Instead, he demands taxes and free money. Taking out of one pocket and putting it in the other creates no additional wealth. In fact, every time it changes hands, it gets smaller. If they want to bankrupt each other to fund UBI payments, I'm fine with that, just don't force me to join. And don't ask government to manage it, they literally can do nothing right.