As far as I know, it will be available "next summer". And only the marketing department of Apple knows (IF they know it already) when it will be a free upgrade to buyers of an Mac and what the upgrade costs will be.

Better ask in one of our Mac forums:
That's where the Apple experts are. Not here in Deals and Bargains. Apple fanboys aren't interested in deals and bargains. They want the best, whatever the price.