Cost: $0.90/L vs. $1.30/L
I always fill up my tank in the US. I carefully ration and add gas so that I arrive in Bellingham with as empty a tank as is safely possible. Gas is just so much cheaper down there.
Last week I spent $43 filling my tank, and the same amount of gas would have been about $62.20 here. (The math is easy since the American and Canadian dollar are at par right now.)
Since it takes about a third of a tank to go to Bellingham and back (140km), it’s not worth going just for the gas, but it works out well for me since my boyfriend lives down there.