Killer Instinct's free to download, but with that ever infuriating Killer Instinct catch 22 payment model of charging for access to characters/content. You've the option to pay to unlock individual characters from Season 3 at $5 a piece, and bundles ranging from $20-60 tops for a Supreme Edition bundle that comes packaged with every character ever released, all costumes, all sets, all perks (exp. boosts, in-game coinage).
Oh, and the XBONE gamers can reap the added benefit of having access to Killer Instinct Classic and the sequel, to boot. Suffice to say, fans seem pretty ready to throw down: