Oil companies have a plethora of market analysis information based on each area's demographics, wealth & population to calculate how much they can raise prices in each region before they lose business. I remember reading an article on this a decade ago. If you think about it carefully, gas prices differ from neighborhood to neighborhood. Sometimes it goes up by a few cents. The cost is calculated to make it not worth your time to drive to get the cheaper gas.
Based on oil company behavior, I doubt a pipeline from Canada would affect gas prices at all. They would just lower production elsewhere. The cheaper raw oil only goes to further pad their pockets.