When assessing the amount being redistributed, much of what's paid in UBI should be netted against what's collected to pay the UBI.
That is, if I am receiving $27k/yr in UBI, but paying $30k/yr in increased taxes toward the UBI, really only $3k/yr is being "redistributed" from me.
This is still going to be a substantial sum, but quite a bit less than "people * UBI amount".
Please don't take this as my asserting that I think $27k/yr would be sustainable in the US - for the record, I don't.
How will willingly pay $5 trillion of taxes after all the hard work only to watch everybody else living of their work for free?
The person receiving $y is actually getting it from the very taxes they pay. So essentially what you are giving them is a tax discount. Except in cases where a person makes $0, for which you have social security anyway.
So this is basically a tax discount? You could simply say based on their income some a% of tax discount will be offered every year.
£1,500 ~= $2,274
$2,274/person/mo * 12 mo/yr * 300,000,000 people = $8,186,400,000,000/yr. $8 trillion a year to provide that level of UBI to all 300,000,000ish Americans.
Not an inconsequential amount of money!