For Xbox Live Gold members, Microsoft offers downloads of Forza Horizon 3 for Xbox One / Windows for $29.99. That's the lowest price we could find by $9, although we saw it in-store for $4 less last December.
Also available for Gold members is the Forza Horizon 3 Expansion Pass for Xbox One for $17.49. That's $18 off list and the lowest price we could find. It includes both the Blizzard Mountain and Hot Wheels expansions.