The challenge with doing the math is the mount of unpredictable aspects of a UBI. We are not good at predicting human behaviors. What happens for those who are

The challenge with doing the math is the mount of unpredictable aspects of a UBI. We are not good at predicting human behaviors. What happens for those who are struggling to survive when they have enough money for a decent living? Will they spend it in a way that revitalizes the economy? Will they use the time they spent struggling to survive in doing other things that make the pie bigger for everyone? What happens for those who were already earning a (barely) decent living? How will their spending habits change? Maybe some of them will choose to contribute to society in non-traditional ways. Would that be good? Will any of them stop contributing to society? We don't know if its a zero-sum change. Its unclear if the pie will get bigger, smaller, or stay the same; if the rich will get richer, poorer, or around the same; if the money we would get from taxation will sustain a UBI in the steady state. The studies on minimum wage increase is the closest we have, but that is unrelated enough that it misses many of the complexities (and advantages) that a UBI could have.