Are there any more creative options which could reduce the cost?
If you are able to get a much better deal to/from Vancouver, and you aren't in too much of a hurry, then going via YVR then transferring to Seattle via Amtrak might be an option.
Apparently the train ride between Vancouver and Seattle is quite scenic in parts, unfortunately the one time I tried to book on the train it was fully booked and I had to go on one of the Amtrak bus services instead. Journey time about 3 hrs or so, including border crossing.
If you have to go by air all the way, then Horizon Air / Alaska Airlines and Air Canada serve YVR to SEA.
Have a great time, however you get there!