People lose sight of the existence of the UBI and how percentages work.
Let's talk about a hypothetical extreme. A living-wage UBI offering $25,000/year, given to everyone, period.
At the same time, income taxes are adjusted so that they are a flat rate of 50% with no reductions or exceptions, that's the effective tax rate. I do not suggest a flat income tax here for policy reasons (though it's much more arguable with the backing of a UBI) but solely because it's just a lot easier to napkin math out, for simplicity. 50% is chosen just for being relatively high; I'd actually prefer a progressive-bracket system with a lower rate for lower incomes and higher for the wealthy, but that's a separate discussion and I'm only noting that here to forestall debating the virtues of this tax proposal.
Someone currently making $25k or less is obviously better-off, but where's the line?
If you're making $50k, you might think "hey, 50% of $50k is $25k, so that's the line, right?" Nope. You're paying taxes on that. I'll be using this for estimates on tax rates, using New York as the state because that was what it was when I opened it up; https://www.talent.com/tax-calculator
So 21.4% taxes, meaning a net income after-tax of $39,300. With the UBI, your take-home is actually $50,000. You're way better off.
$70,000? Right now you pay 25.2%, so your take-home is $52,360. With the UBI, $37,500 + $25,000 = $62,500. Still way more.
$100,000? 28.4% tax rate, so net take-home is $71,589. With the UBI, $50,000+$25,000, you're still better off at $75,000.
It's somewhere up around a gross income of $150k before the UBI's high tax rate actually translates to lower take-home.
Again, hypotheticals used to demonstrate a mathematical point, not as a firm policy recommendation.
Edit: The point is, focusing on the "costly" part is a distraction. It doesn't matter how much money travels through government hands. What should matter, at the end of the day, is individual take-home income, after-tax. Pushing more money through the government's hands resulting in more money for the vast majority of citizens, that's not "costing" those citizens in any appreciable sense. Budget size doesn't matter. Ensuring revenues line up with expenses, that is what matters.