the price or 91 octane is usually 10-12 cent/L cheaper then average, that is around 40 cent/gallon. fillup a few times and it save enough to pay for my costco membership card. so I try to fill up at my local Costco whenever possible, but there are rules... only when
1, my the tank is near empty, 2, the line is short, 3, not between 9-8pm no weekend. no reason to drive 15km (round trip), idle in line doing nothing but burn fuel but burn fuel, to save $5.
having say that, it still can't beat the feeling of filling up at south of the border in Buffalo NY.