If we left we would take our 25% of the UN revenue, Britain would probably leave also taking their 6% and Japan taking their 13% and Spain taking their 6%, that's a 50% reduction their their operating expenses right there. In addition we provide 35% of the UN military budget (actually it's more than that because we furnish military equipment to many of the other nations and logistics for all of them).
We also provide 60% of their humanitarian aid.
If we leave there is no UN. If the remaining countries (Russia who provides 2%, China who provides less than 1%, France 6% and Germany 6%) who's going to pay the light bill? Who's going to pay for all those useless UN employees?
You forgot finding new office space as well once Derek Jeter is hitting home runs at New Yankee Stadium in the UN Office Park.