I have been buying gas at Costco for over a decade at the Eden Prairie location. Last Saturday it was 2.75 and everywhere around me was 3.10, .35 cent difference or about 8.75 savings on a tank, I save enough to cover the membership in 6-7 tanks.
In addition over the decade I have been buying gas at Costco I have seen the difference between Costco and the area around me vary from as low as about 10 cents to as high as 45 cents a gallon.
I believe Costco just marks their gas up using a cost + formula, not what the market will bear like normal stations.
I commonly refer to the difference between the 2 as the SCREW FACTOR