Cash fares from PDX - BZN are rarely cheap. I would say $100 each way would be about as low as it will go. More likely $125. If you have Alaska Airlines miles or Avios, this could be a great opportunity for finding saver space and using miles. PDX - BZN can go as low as 5K miles each way using Alaska miles, and I believe 7.5K avios in each direction. You mentioned you fly BA, if you have Avios, I would suggest this is probably a decent time to use them, I believe you may have to call BA to book. There is currently not low mileage award space on the date you are looking at, though I would guess it is very likely it will open up. Enjoy your trip.