If you catch the light rail it would cost $2.50, buy your ticket at the automated kiosk. Trip takes 20 - 30 min. Depending on time of day and direction traveling. If you want a cab, I know it's $40 from downtown to the airport. I'm going to say it would stand to reason the reverse trip would be the same cost. Hope that helps.
About $2.50 from seatac or little more
Reserve 45 minutes between baggage claim and exiting westlake station
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