I always assumed it would be something like earned income tax credit, or supplanting existing welfare models. I don't think your arithmetic accounts for that, and assumes that every working class person would receive a UBI payment, but maybe I'm reading it incorrectly. Like, why would I, for instance, with my full time job and decent income, get a basic income on top of that? The answer to your question, for me, has always been: I would get taxed and it would go to those out-of-work or under the poverty line, and I would be able to take advantage of that if I were in either of those two categories.
You can go one of two ways: You can give it to the people who can't earn enough to not need it. let's make it a nice livable number like $30k. That's half again more than SSDI, which averages closer to $1100 a month. I know a guy trying to live on $1100 a month; it ain't pretty but regardless, where does the cutoff happen? 'cuz it's gonna be a cutoff. And at that cutoff, there will be a big angry discontinuity between "low-end job" and "job not worth taking" which will of course go to non-citizens who can't qualify for UBI. If UBI gets you $30k a year, what kind of job would you take for 38? 40? 50? 70? If you can scrape by doing nothing for $30k or work 90 hours a week filing TPS reports and spending three hours in traffic for $60k, are you still going to do it? How 'bout $90k? How 'bout $120k? Or you can go like the Alaska Permanent Fund where everybody gets $2,072 a year. And where everybody pays $9 for a Big Mac combo meal. The APF basically pays the price difference of a big mac a day... much like the price of a popcorn and a coke is pretty much the price of a movie ticket. Your understanding of UBI isn't dissimilar from welfare - we make enough to not want to qualify for it. But we can all agree, I think, that UBI is supposed to be more than welfare. It's supposed to be more than the pathetic food stamp existence we expect our poor to grovel for. And if I'm an employer paying my employees $50k a year, and the government is suddenly giving them $30k to stay home, how can I not be expected to raise my salaries to $80k a year? And by the way, I'm paying tax for that $30k to stay home, both business and personal, right? So really, if I'm an employer I'm now paying an additional $60k to keep every $50k employee working. UBI is a system of wealth redistribution. I think everyone can agree on that. it's the nitty gritty that gets ugly, and that's usually where the UBI proponents start shining it on.