Would Keystone XL, with branches into the Bakken field, reduce the costs per barrel?
The Keystone XL would reduce the transportation cost per barrel.
Oil is a fungible global commodity. Given the quality, the refinery doesn’t care where it comes from. They are not going to pay over market price and the seller is not going to accept less than market price.
Cheaper transportation means less cost to the seller, ie more dollars per barrel for production. New marginally economic projects become profitable and investment is made. More jobs stay local and more dollars stay local.
Eventually, additional production on the world market should lower prices, but changes in demand with a better economy may offset the lowered production/transportation cost.
OPEC could decide to reduce their production to offset new production in North America. That can hold the price at the same high point. But if we pay the same price today and lower the dollars that leave the country, have more local jobs, I call that a win.