I've gone through a bit more of the data and it's definitely the rental-car fees that pushed Santa Clara to the top of the list.

Remember, this is based on what 15 million users reported in 500,000 individual expense items in the year 2011. It is not an attempt to compare costs that you might individually incur if you price shop.

The rental car data was as follows:

Santa Clara - $334.55
NYC - $154.19
San Fran - $237.31
Boston - $198.40
Garden City - $196.92
Plano - $226.39
Bellevue - $231.97
Chicago - $188.60
DC - $ 151.60

Santa Clara also had the second highest ground trans. costs (Garden City was the most expensive). For lodging, it was nr. 7 out of 10 with a cost of $104.34 versus the most expensive, NYC, $196.57. At $35, it was the least expensive for dining (NYC was the most expensive, $67.04). Santa Clara was also 7 out of 10 for entertainment costs.

The study makes no attempt to analyze any of these items so there is no explanation as to why rental cars cost this group of travelers so much.