US Citizens: Couldn't find a readily available reliable source, so I'll use residents instead. Should be close enough for this math though I'd imagine this would be more likely a citizen program rather than a resident program.
US Residents: ~325 million
Federal Tax Revenue 2016 - ~3.27 trillion
Federal Tax Revenue - ~272 billion / month
Federal Tax Revenue - ~$850 / resident / month
What if we didn't give it to minors?
US residents over 18 - ~247 million
Federal Tax Revenue - ~$1100 / resident-over-18 / month
That's if we spent it all on UBI, no debt service, no federal employees, no military, no infrastructure spending, no social security, no pensions (even for federal employees who earned them and are legally and morally entitled to them), etc.