At $1500/mo for everyone in the US, that comes to just shy of 6 trillion USD. For comparison, the total federal tax revenues for 2016 are going to be approximately 3.5 trillion USD, including payroll taxes.
At $1500/mo/person, this one program would present 5/3rds of all federal tax revenues.
A more realistic figure is probably much closer to $650/mo, assuming you take ALL of the current federal spending on healthcare, social security, veteran's benefits, pensions, and welfare and direct it into UBI. This says nothing of the legality of cancelling existing or contractually promised federal pensions, nor the morality/practicality of changing the existing social security and Medicare systems, of course.
$650/mo/person still leaves us with a deficit (as today) and is a pretty meager existence for sure!
Nevertheless, the money is going to come from somewhere and that somewhere is taxes. It would be unreasonable to not calculate the total sum of taxes required for various levels of UBI funding, IMO.
It is the same way infrastructure investment works. Nobody wants to do it because the upfront bill looks big. But the decades long projection of efficiency of most high-demand projects would eclipse the cost by an order of magnitude. Meanwhile special interests (mainly those who would not immediately benefit from the infrastructure themselves) keep phrasing the conversation around "how much it will cost" these projects never get done, and the national economy suffers as a whole for it.
If not, you need to plan spend less than double what you're currently spending.
This is the reason I think UBI will be disastrous, but it's going to take a generation or two for the rot to set in.