Retail is $ 19.99 so 75% off for a great game! Not sure if it’ll be compatible with the halolens when it comes out and just an FYI, I did NOT own the 360 version so it’s not an add-on or upgrade.
P.S. I do not have a URL as it’s showing the full price on xbox.com. This may be a Gold only offer, if you see it for non Gold let everyone know!