SEA-BZN is consistently on sale for $69 saver and $89 main cabin, PDX-BZN is usually similar. I expect those sale prices will be extended to the end of the month of September over the next 2 weeks, as others say, those low cost fare buckets arent open yet and this is fairly common with Alaska Airlines flights that are out near the end of schedule.

First class can be a real bargain on this route, and the 1-2 seating is nice. We snagged flights for $125 each way in F for a long weekend later this winter.