Trillions. That's what's currently going around in quantitative easing. That's why many are calling for QE for the people. Instead of giving money to financial institutions, give it directly to the people.
UBI isn't like defense spending. It isn't a black hole you put tax dollars down and get nothing out. It means tremendous goods demand for the necessities of life that is extremely stable and reliable for businesses to profit off.
Unlike (some) defense spending, UBI doesn't intrinsically create some economic value. Increased stability would indirectly create economic value, but it needs to be more than the face value of the UBI check for it to not lead to gross inflation.