At the moment it's at no extra cost. You already pay for Prime - it is not free. But for the time being you'll be getting it on top of the things you subscribed to Prime for.
When/if the Prime sub rises you can push the theories.
BT got football for free - price rise
Virgin got BT sports included for free - price rise
You'd have to be utterly stupid to think this is not going to increase the cost of Prime.