Some things to consider:
Do we really WANT any of their money? It would be like admitting that we need the UN, that the UN is actually more powerful than us. Besides, we can do without it, and there are other countries that need it far more. It's kinda like welfare: if you can pay for it yourself, don't make the goverment pay for it for you. If we can fix this problem ourselves, we shouldn't needlessly waste funds that could go to more dire recipients. The money we would take from the UN would have to be taken out of other potential aid plans, and we'd just end up picking up a part of the tab for those same programs down the road anyways.
On the flip side, the extra $$$ certainly wouldn't hurt (financially). It'd also be nice to see our investments in the UN finally have some sort of tangible benefit. If we get no financial aid from the UN, it'd be like paying for expensive car insurance that won't give you money if something happens to your car.
I can see both sides on this issue.