This is basically a marketing move: HBO gets to keep their new programs so customers have to have HBO to see GOT and so forth (HBO gives up its back catalog but I doubt their old shows bring in new subscribers so that isn't a huge loss), Amazon justifies their Amazon Prime price increase and also gets people to buy Fire TV (Amazon Prime is probably not coming to any other device like Chromecast). I would say it's a huge win for Amazon and a so-so win for HBO, so likely Amazon is paying a hefty price to HBO for this ad campaign.