As expensive as Game of Thrones is to produce, it is still not coming close to Friends, believe it or not. The long-running sitcom cost upwards of $10M per episode in its final season, simply due to rising salaries of the six principal actors. I assume this will not be an issue with Game of Thrones, where characters have a habit of dying horrible deaths quite often.
Mokalus of Borg
PS - Then again, there are plenty of GoT characters who probably won't die.
PPS - To which George RR Martin obviously replies: "Challenge accepted!"