A good place to find information about the median income for various places in the United States is the US Census Bureau’s website. The Census Bureau collects a great deal of information about places in the US and posts many valuable statistics on their site. Below is a link to their page on Santa Clara County.
Santa Clara County is one of the richest counties in the United States. In the link below, you can see that Santa Clara County has (or had, from 2009 to 2013) a median household income of $91,072. This is about 33% higher than the median household income for the state of California as a whole, which was $61,094. According to the article in this link, “Santa Clara County has the highest median household income in the nation.”
For the United States as a whole, the median household income is $53,046.
Please note that, while people in Santa Clara County make a lot of money, it is also true that the cost of living there is higher than in many other places. This may mean that people in that county are not necessarily richer (compared to their costs) than people in some other places.