Upvote:7
How can I easily compare the affordability of two departure airports ...
By looking at prices on Google Flights for durations, dates & booking classes that are representative of your specific flight plans, populating a spreadsheet, and looking at the results.
Sorry, I'm not trying to be snarky here. Google Flight's "price on calendar feature" and specifically the prize graphs are great tools to effectively display a large set of flight prices and quickly distill it to a single number. I just looked up SFO/SEA to TPE/HKG/MNL for Jan + Feb, and it took me less than two minutes, a lot less than writing this answer :-)
If you want to get really fancy and find the absolute lowest price for both airports at the same time, you can use ITA Matrix instead, but it'll take a you a few minutes longer.
FWIW: I didn't see any meaningful difference in my quick research. Differences seemed less than 10% and just "noise".
There is more than just price to pick a "preferred" airport.
Needless to say: housing is substantially more expensive in the Bay Area, so unless you fly a whole lot AND pay everything out of your pocket (and not on company/business dime), the difference in actual ticket prices will not make a meaningful difference and it feels like the tail wagging the dog.