Gas prices in San Luis Obispo County are the most expensive In California and the country. Currently, SLO County residents are paying 3.68 a gallon. That’s higher than Maui, Santa Barbara, and San Francisco. It’s not the first time San Luis Obispo has topped the list, just last summer the average price for a gallon of regular was 3.91… It could get worse. Experts say this week’s refinery fire in Carson could push prices up another 10-to-15 cents a gallon.