And of course the US doesn't actually have 300 M people over 18, 20% are under 16.
The number is feasible for the US, though it would be one of the larger spending programs ever conceived. But not impossible.
Social security is not means-tested welfare.
Why do you presume that? Its common for UBI advocates to advocate it (at least as it matures) replacing means-tested benefits; its a lot less common for them to advocate it replacing other things, including earned benefits.
But just from a cost standpoint, it seems unlikely that we could actually afford a UBI would that actually provides universal-net-income. In practice those making above a certain threshold (from actual work, investments, or pensions/social-security) would have some kind of claw-back baked into the tax-system. The system still needs to be universal, everyone gets UBI payments, so that we don't cause the perverse effects of current means-tested programs, yet in practice, many individuals would just pay-back the UBI in taxes.
The point of UBI isn’t provide a universal net income, that’s plainly impossible (at least, from first order effects; after factoring in induced economic growth, it may be possible, but it is still not the point.)
The point is to eliminate the welfare trap posed by rapid early clawback of outside income in means-tested welfare programs, and eliminate the inefficiencies of redundant tax and welfare bureaucracies. If you tie in taxes on capital income as a progreasive taxes on non-capital income, it can also be a way to mitigate inequality driven by progressive automation and other factors driving returns to capital and a narrow subset of labor.
No.
with P as “person(s)":
300MP × 1k$/P/mo = 300G$/mo
300G$/mo × 12mo/yr = 3.6T$/yr