Gas Smogs Costco Results Costco Wholesale ( COST ) was a big drag on the index, Perkins says. Gasoline prices have risen this year, but they're still far below that of a year ago, and that hurt Costco. "Plugging in" Costco's core U.S. same-store sales tally in May, same-store sales resulted in a 3.8% rise that month instead of only a 0.6% gain, Perkins says.