Currently the cheapest for both the game and its DLC packs - from an official source too.
The Expansion Pass contains the two game-expanding DLC packs of Hot Wheels and Blizzard Mountain.
- Forza Horizon 3 standard version (Full Game) - $13.18
- Forza Horizon 3 expanasion pass (DLC) - $15.73
The game is also playable on the PC, with a ~51.41 GB install size.