How much money has the US spent on Space activities?

The total amount that NASA has been budgeted from 1958 to 2008 comes to around $416 billion dollars, an average of $8.2 billion per year.

When measured in real terms (adjusted for inflation), it is closer to $790 billion, or an average of $15.8 billion dollars per year over its fifty year history (According to the Office of Management and Budget and the Air Force Almanac).