[hypersoar]

First of all, you're only going to pay the adults. That eliminates about a quarter of the population. Second of all, you're still going to collect taxes, so many people will just have the income taxed back. Say, only people below median income end up keeping the money. This cuts the figure down to 1.44 trillion. This is still a lot (we currently spend about $850 billion on social security, which would presumably be replaced by basic income), but it's not unmanageable.