Here's the spending and revenue for the US, 2013.
With universal basic income, you would be able to remove Family and Children benefits and Unemployment benefits from spending. You'd also be able to reduce pensions and disability benefits by an amount equal to UBI.
However, a basic average guideline for the food stamp program will show that an average family of 4 can expect an amount up to $500 per month for food stamps. This figure will greatly vary based on the age of the family members and medical needs. A single person household will show an expected average of up to $200 per month. Again, these figures are averages and not state specific.
Cash allowance benefits for financial assistance will also be state regulated and allowances paid will also vary based on different criteria. However, an average expectation can be placed on a family of 4 receiving up to $900 for their TANF allowance. A single person household can expect an average of up to $300.
This would be the equivalent of giving adults $500 per month and children $200 per month or $6,000 per year and $2,400 per year respectively. According to here, 23.1% of the population were under 18 in 2014. That gives 73,088,400 people under 18 and 243,311,600 people 18 or over. If everyone 18 or over got $6,000 per year and everyone under 18 got $2,400 per year, this would cost $1,635 billion per year. By removing Family and Children benefits and Unemployment benefits from spending you'd save $427 billion. According to this site, there were 1,299,761 people under 18 and 7,035,943 people over 18 claiming SSI at the end of 2014. Since those people would be getting a UBI, their payments could be reduced by the amount of UBI. For under 18s that would save $3,119,426,400 and for over 18s it would save $42,215,658,000 giving a total saving of $45.3 billion.
So the cost of these changes would have added $1,162.7 billion to government spending in 2013 taking it from $6,109.6 billion to $7,272.3 billion. As a percentage of GDP, that an increase from 36.67% to 43.64%
That's totally feasible.