American Truck Simulator will include a free DLC region at launch, while one more region is also currently planned to be release for free at a later date.
SCS Software have mentioned multiple times that American Truck Simulator would launch with the state of California as the base map. What they have announced today is that the region of Nevada will be available for free on February 3rd when the game releases on Steam. It will not be part of the base game, but as an update for people who have bought the game.
They have also detailed future plans for DLC. Arizona will come as a free update at a later date, with no specific time being mentioned. After the Arizona expansion the majority of the map updates will be in the form of paid DLC, and how far they go with the world after release will depend on how well the game does financially.
American Truck Simulator will release on February 3rd worldwide, and their recommended retail price is set at $20.