People will either do the job or you have to pay a reasonable amount for it - one key principle of the UBI - more people get a choice how they use / sell their time and are not prey like today in many regions / countries to the few who dictate how much they will receive for their time.
£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!
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.
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.