$0, but it's not likely to happen anyway... Selling an ad-free version of their services would undermine the value of the audience presented to their advertise

$0, but it's not likely to happen anyway... Selling an ad-free version of their services would undermine the value of the audience presented to their advertisers. It would also be tantamount to admitting that their ads are annoying, if even in a minor way, and this would contradict their efforts to deliver relevant ads unobtrusively.