Stockton and Tracy were among the least expensive cities in California, but are still respectively 12 percent and 24 percent more expensive than the average of 273 U.S. communities in the 2015 cost of living index from the Council for Community and Economic Research.The Center for Business and Policy Research at University of the Pacific, which collected cost data for Stockton and Tracy, said Tuesday that the widely used index drives most popular online cost-of-living calculators.