note that most of the stuff from ND and western Canada is headed for Chicago area, WY, CO, Utah (and eventually CA), and southern Illinois/Indiana rather than Cushing.
i managed to lose the map that showed the discount to destination relative to WTI from western ND but whenever the map was made (I'm going from memory) it was ~ -$3.5 for Billings or Casper; ~ -$2 for MN/Chicago ; 0 to Cushing; and ~+$3 for St Charles. Obviously prices are now different but the general scheme is the same. I believe the oil is paid for at the refinery and all of this is part of the reason for not investing in refiners (as an oil company or an individual)