Virtually every discussion of basic income on the internet these days avoids doing even the most basic arithmetic. So I'll give a very oversimplified set of calculations below. The details can be nitpicked ad nauseum, but this is essentially Feynman-style estimation, so please see the forest and not just the trees.
First, some facts:
According to the BLS, the lowest quintile of Americans spent $12,955 on goods and services in 2014 [1].
The US adult population in 2014 is about 242.5 million people [2].
So if we wanted to provide a guaranteed unconditional income equal to the average yearly expenditures of the bottom quintile of Americans, the cost would total about 3.14 trillion dollars.
Total U.S. tax revenue in 2014 was 3.2 trillion dollars [3].
So a basic income program that merely provides poverty-level income for all Americans is going to cost all of our current federal tax revenue, and we are already running a federal defecit.
This is plainly not feasible without vastly increasing tax revenue.
You could argue that state and local revenue could pick up the slack, but states and municipalities are generally under severe budgetary pressures as well and have even less power to raise additional revenue. Even if all tax revenue in the US from any source is considered accessible for a basic income program, that pile of money is only about $6 trillion, so a poverty-level basic income program is going to consume half of all tax revenue collected from any source in the US.
All of this is an optimistic calculation. It doesn't account for the contraction of the economy (and thus tax revenue) that would occur due to people ceasing part time work or other low paying jobs. Or the increasing amount of federal revenue directed to servicing our national debt [4], which will make any basic income system increasingly unaffordable in the future (absent corresponding economic growth). You could alleviate some of that economic contraction by taxing the basic income itself (possibly conditionally, based on economic circumstances), but then you are providing a post-tax basic income that doesn't even meet the poverty level.
The fact is that the demographics of the United States simply prohibit any meaningful basic income system from working at the present time. It simply won't work without increasing tax revenues to levels that will destroy the economy. And this is ignoring the political obstacles that would have to be overcome (such as placing most of the additional tax burden on the 1%).
Our existing welfare systems, inefficient and bloated as they are, at least attempt to target those who are most at need. If you try to make a basic income system that essentially replaces existing welfare programs by taxing most of the basic income back from people who exceed certain economic threshholds, you are doing little more than inefficiently adjusting tax brackets.
It's a nice dream, but somebody has to pay for it and right now nobody can.
[1] ftp://ftp.bls.gov/pub/special.requests/ce/standard/2008/quintile.txt
[2] http://datacenter.kidscount.org/data/tables/99-total-populat...
[3] http://www.usgovernmentrevenue.com/fed_revenue_2014US
[4] http://si.wsj.net/public/resources/images/BN-GT738_ristid_G_...