I live 2.5 km from the Sumas crossing…I only cross when there is no line up. I save about $10 per fill up on my car, $15 on the wife's. So that's $25 a week/$100 a month on fuel…which I now spend on the Port Mann Bridge.
As for bringing gas back I asked the border guards one day and here is their answer:
You can bring back your vehicle's tank duty free, technically you can't bring anything in a jerry can with out being charged duty/tax/what ever it is.
Now the bigger problem is the transportation of the jerry cans. Here is a quote from Transport Canada:
In Canada, even when gasoline is transported for non-commercial purposes or for personal use, the Transportation of Dangerous Goods Regulations still apply. This differs from the United States, where dangerous goods are only regulated when in commerce. However, the Regulations contain a number of exemptions, allowing gasoline to be transported without having to comply with all parts of the Regulations. There are a wide range of exemptions, each providing different levels of requirements. Some of the most commonly used exemptions for transporting gasoline include:
150 kg Gross mass exemption (section 1.15 of the Transportation of Dangerous Goods Regulations): The 150 kg gross mass exemption is the most commonly used exemption under the Regulations. In fact, most people use this exemption without even knowing it when transporting jerry cans. It can be used when you transport 150 kg or less total mass of gasoline, in containers that are 30 kg or less.
Found a number that says gasoline weighs approximately 0.76 Kg per litre. So that puts 150 kg's at 197 litres of fuel to carry under the exemption.
And a news story from last year: