So how much does Uncle Sam give the States?

According to this great article on the coastal sand mining business (everybody's favorite topic) in Korea, state governments rely on the national government for over 40% of their budget. This severly limits their independence, which is the whole point.
I've been trying to find out what the situation is like in the United States, without much success, especially since the past year our so have been highly abnormal due to stimulus funds related to the recent finanical crisis. Can anyone point me in the right direction here to find out, on average, how much of U.S. state budgets are supplied by the federal government?

So how much does Uncle Sam give the States?