I used to believe this as well until my company starting asking me to consider options on my very regular flights. I quickly discovered quite the opposite now that the government(s) and airline(s) have imposed so many taxes, levies, surcharges, etc. I almost ALWAYS fly cheaper out of SEA than YVR; significantly cheaper.

For example, next week I have priced YVR-ORD-YOW-YVR. That quoted back to our company at >$2300USD. But when I priced SEA-ORD-YOW-SEA, it came back at $1300USD. It is cheaper for me to rent a car, drive to SEA, leave the car in the parking lot, pay the parking, drive home and pay gas, than to fly out of YVR! AND, as a bonus, I can maintain my policy of avoiding AC skin at almost all costs!