So let's just assume that (generously) the cost here is really just "give the second income quintile an additional $1,000 per month"; on the rough-and-ready bas

So let's just assume that (generously) the cost here is really just "give the second income quintile an additional $1,000 per month"; on the rough-and-ready basis that most transfers are currently going to be the first income quintile and that UBI offsets those. ~260M adults in the U.S. so ~52M in that group; for a cost of $624bn annually. Bear in mind, this is assuming no-one in the third or fourth quintiles get anything, which is much less generous than your proposal, and that $1,000 per month is at the low end of the range often proposed. And let's say we're going to fund that by taxing the top 25% of tax payers. In 2019, last year with good data, the top 25% paid about $1.4tn of income taxes. So basically income taxes would have to increase by ~50% for that group.