1 out of 25

http://www.joshuakennon.com/1-out-of-every-25-households-millionaire-united-states/

Probably a lot higher in SD county.

5. San Diego County, California: The fifth county in the U.S. and the 3rd in the State with the most number of millionaire households, San Diego has 102,138 millionaire households that make up 9 % of such households in California. It also accounts for 1 % of the total millionaire households in the country.

http://www.streetdirectory.com/travel_guide/73297/real_estate/top_10_us_counties_with_the_most_millionaires.html