Southern California home prices and sales increased in September at the fastest clip of the 1990s, according to a report by a real estate data firm. The median price paid for a home in six Southern California counties surged 6.8% to $172,000 last month, from $161,000 the previous year, according to La Jolla-based Acxiom/DataQuick Information Systems. Los Angeles County reported the biggest price gains, with the average home selling for $174,000, or 7.4% more than what it sold for in September 1996. A total of 8,224 homes were sold in the county in September, 18.5% more than the 6,942 sold last September. "These higher prices should bring more homes to the market. We expect sales to stay higher for the rest of this year and well into 1998," said Mike Ela, DataQuick president.