Currently from the tax money used for social benefits about half ends up paying for the bureaucracy. Thats crazy. Using UBI we get rid of this overhead and can use almost 100% to pay out people.
In the current system, you would simply cut all the different ways to get benefits, and simply give everyone the money. This would be financed by an increase in taxes so that for working people not much changes. What they loose in extra taxes they get back in UBI. This would not be more expensive than the current system and is a huge gift for work heavy industries since salaries drop. I think this would be much much better than what we have right now.
Still I think that tax-funded UBI is fundamentally misguided, just as the current welfare state. Instead I think that each human has an equal right to benefit from the land and resources of this planet. This could be represented by a UBI financed from resource sales, ground taxes, ... . Then there is also no question how much it should be. It is simply what it is.