IIRC, the UN decides what funding it wants, and then demands member states pay X% of that number.
A little investigation reveals that the USA pays 22% of the UN budget.
Under the 2015 “assessment” that is $654,778,938.00 thee largest “payer” other than the USA is Japan at $293,960,45400.
For reference China the #2 economy pays $139,694,305.00 roughly 1/5 of what the US pays, 1/2 of what Japan pays with an economy that it 3X Japan’s.
The United States should not be paying one thin dime to the UN.