The Tomb Raider series definitive editions are currently discounted on the Microsoft Store. Apologies for the rough prices but I came across them last night:
Tomb Raider: Definitive Edition - £4
Rise of the Tomb Raider: 20 Year Celebration - £7
Shadow of the Tomb Raider: Definitive Edition - £16 (or the 'Definitive Edition' upgrade alone is £6)
These are all 'with Gold' prices.
EDIT: Seems it's part of a bigger sale:
https://www.xbox.com/en-gb/promotions/sales/ultimate-game-sale