If you sniff around, you can find numbers for it, but I don't have the bandwidth right now. Contrary to what Martinuzz said, the hard-core experimental libertarians are totally on board with (at least hypothetically) carving out the entire entitlement sector of the government, taking the savings, dividing by population, and mailing checks.
Edit because fuck my data cap and the book I'm trying to read:
http://www.libertarianism.org/columns/libertarian-case-basic-income
Has the point at 20,610 each a head, which definitely isn't chump change for a purely budget neutral measure.
Contradictions for second and third order effects, money yoinked from State budgets, and kinda-sorta breaking capitalism notwithstanding.
That number would roughly turn into $396/week and 9.91/hour untaxed if put in the context of a 40 hour job. While it would likely wind up lower, it's not really that bad of a result. Not sure how badly it would break capitalism, though. While the motivation to work to simply not die would dissipate, I don't think people would want to live at that extremely low level when getting a job could make a pretty major increase to their income. Might even help remove a fair amount of crime hindering capitalism, with the individuals motivated to do so in order to satisfy bills no longer needing to. Granted, this doesn't account for greedy pricks raising the rates on utilities and similar.
You'd obviously need to couple it with stringent anti-trust and anti-abuse regulations, which is mighty difficult in post-Reagan America. Not to mention that most city/county utilities are already firmly entrenched monopolies with existing habits of screwing people over just because they can.
You're also relying on the assumption that the bitter old people who want their turn on the SocSec train (and whom are represented by one of the three largest lobbies in the country) would let you pull the funding. Pay-forward systems are fucked both ways, since they collapse like any other pyramid scheme as soon as the newly-entering population begins to decline but also screw the people who bought in relatively early. If there's one thing that'll get old folks out on election day, it's a threat to their retirement funds.
But yeah, that's the fundamental idea behind basic living stipends and such, they're supposed to be enough to meet very basic needs while leaving luxuries, better basics, &c. as motivation to work. Nobody
wants to live on $300-400/wk, but you won't starve on it either as long as you're not trying to support a family.