Being the researching (see obsessed) car guy I am, I was thinking a while back about how you'd replace the spark plugs in a BRZ/FR-S given its engine configuration and the size of the vehicle. I decided to follow up with my local Subie and Scion dealerships and the cost to replace the plugs is: $550-600 (CAD)

Keep in mind that this is right from the dealer, and will take 4 hours of work as the process is quite involved (apparently the engine needs to be lifted or specially trained "little people" need to do all the work).