Whats the cheapist place to fly to in europe from Seattle ?
Where is the cheapest place to buy gas in Point Roberts? The answer is the same. It depends, it varies. London tends to be more expensive but it really depends on who has a sale or promotion, time of year, scheduling pattern, etc. And part of the problem is Seattle. You don't have the save level of competition as New York or SF. You question has no answer.
I would that if you are in the Military it would be anywhere that MAC could take you. (or at least that's what we called it a few decades ago - don't know if that's still the name of )