Sour crude is typically cheaper than light sweet crude. There is a glut where Keystone dead ends, and that region typically has a little better price than other regions. We are in the process of retooling our refineries to process our light sweet crude. Our policy after WW2 was to import Mid East oil and cut back on drilling here.
Now we produce so much here, we would do better to refine it instead of exporting it.
There is no need for the Keystone XL, as all oil is traded on the world market. As long as their crap gets to market, it doesn't matter what path it takes. XL is primarily for tar sands. The Keystone already makes it all the way to Texas.