CoreLogic research analyst Andrew LePage said that the number of deals last month for less than $500,0000 dropped 28 percent in a year and the number of deals for less $300,000 plummeted 41.5 percent.
A report by Metrostudy shows that only 12 percent of new homes are priced under $500,000 because prices in suburbs are rising.
"Many first-time buyers face a daunting challenge in one of the nation's priciest housing markets," LePage said in a statement.
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Greg Gross, Metrostudy regional director for Northern California, said in a statement, "The story in this market continues to be affordability."
Last month, the median price for new and used homes in the Bay Area was $739,000, down from $740,000 in August 2017.
The median price last month is up 13.7 percent from $650,000 a year ago as buyers bought in areas with higher-priced homes, according to LePage.
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The all-time high median price is $775,000 set in June of this year.
Last month's sales of new and used homes dropped 7.5 percent from 7,934 to 7,338 in a year. Last month's sales were also 11.7 percent lower than the historical average for September.
Historical data for the report start in 1988.
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