That's just over $10,000 per citizen. Is that even a subsistence wage?

It's closer than you might think. If you have a wife and two kids, that's $40,000... That's only a little under a median household income. Perhaps kids pay out less, but that raises the amount available for adults.

If you're single you pick up a couple roommates, just like people working minimum wage jobs already do.

Also, the amount paid out is related to income. If you implemented a $10,000 UBI, plus a 25% universal flat tax, you would only receive the full $10,000 if you had $0 earned income. If you're earning $40,000, your net UBI/tax is $0. With $80,000 earned income you'd have $10,000 net tax. So you're not paying out $10,000 to every citizen.

With the nice round numbers above and $57,220 income per capita, the average tax will be $(57,220 - 10,000) * 0.25 = $11805. That results in 20.6% revenue/GDP. That's a little high, but it's completely plausible. You can nudge the variables a little and end up with a very reasonable scenario.