Actually, how I would implement it is as a refundable tax credit in place of the basic deduction. In other words, if you have no income you receive your full UBI. However, you are taxed on your first dollar of income (rather than getting a tax free threshold as you do today). There is still an incentive to work as you still get to keep 75% (or whatever) of every dollar you earn on top of the basic income, but at some point the amount of tax paid exceeds the amount of UBI received, and you become a net tax payer.
EDIT: The point is that by working the numbers, you can have everyone wind up in roughly the same financial position as today except without the overhead of the various social programs.
I might be persuaded to do that, if it meant that pretty much all social programs go away, and that a lot of laws, agencies, regulation, subsidies and even military spending were cut. With a target of cutting the federal budget by 70%. If course it feels like I have that much coming out of my paycheck already.
1) Even if the marginal tax rate is fairly high, your average income earner will be getting that $18000 back in UBI which makes the effective rate in your scenario more like 5%.
2) The majority of tax revenue comes from the highest earners. A quick google gave me this (from https://taxfoundation.org/summary-latest-federal-income-tax-...):
The top 1 percent (1.3 million filers) paid a greater share of income taxes (37.8 percent) than the bottom 90 percent (124.5 million filers) combined (30.2 percent).
In other words, I don't believe your math is a reasonable approximation of how the budget would work out. If everyone pays the same effective rate as today (including welfare and other social program recipients who can be treated as having a negative effective rate), nothing changes. Also, there is no need to presume a flat tax rate; the U.S. does not have a flat tax rate, now.
EDIT: Also the average household size in the U.S. is 2.53 people, not 4. A naive calculation would suggest that a household of 4 should therefore expect a mean income around $83K.
No, of course, I would not propose that. There are many ways to handle it that effectively amount to the same thing. The UBI could be paid out monthly or fortnightly or whatever seemed good with the tax situation otherwise being the same. Your employer would have to withhold a bit more tax to compensate, but the end result would be the same.
The point is not to argue specific implementation details; it is to offer another way of thinking about it.
That said, theorycrafting isn't numbers.
Today the US population is roughly 350 million, and we spend 927 billion on social programs between state and federal. That works out to $2800 and change spent per person. (assuming my numbers are correct) That's, as you say, not nearly enough.
Okay, perhaps we can say some large portion of that population is children that would cost less. also families can benefit from shared housing. So let's switch to a theoretical family of four model. For ease of calculations, let's assume large families and small families balance out: $11,200 per (theoretical) family of 4...which isn't the individual poverty line.
So....yeah, you seem to be totally correct. Which makes me wonder what numbers those libertarians in favor of UBI are using?