US military spending = $1.5 trillion per year (~$700 billion for the DoD, the rest from other military and NatSec programs)
US per capita healthcare spending = $8,600/person/year (edit: corrected by caseysoftware below, that includes private and public spending)
Social Security = another ~24% of the federal budget
Interest on the national debt = 7% of the US budget
Again, rough numbers off the top of my head, but you could get a decent basic income going across the entire US just by switching to an affordable healthcare system and cutting the military budget to something on par with what the rest of the world spends. UBI would also replace and simplify a lot of the existing welfare and social security system, so that portion wouldn't even be additional spending compared to what's already happening.
The obstacles seem more political and cultural than fiscal and economic. To a large extent it's basically wanting to replace the current corporate welfare for the military-industrial complex, healthcare, college bubble, etc. systems with something that would (arguably) benefit society far more.