Well, we're imagining a world where robots have made us vastly richer. (Self-driving cars! 100% automated factories!) Stark increases in inequality will also mean that more of the US' total income is given to the highest earners, i.e. those with the highest tax brackets. So we'd collect quite a lot more in taxes even if we left current rates completely unchanged.

If robots don't end up making society incredibly wealthy and more unequal then there's no point in UBI to begin with. So it doesn't make sense to simply look at current figures.