As the Death Star is located in space, the only means of transportation is by using a starship. Travel distance and time will depend on the location of the start point. Costs will vary on the rental price of the starship.
If you have more time, consider adding a visit to the throne room and explore the Emperor's personal chambers. Alternatively, you can subtract the Power Station visit from the itinerary if you prefer more leisurely pace. Be sure to check the Death Star opening and closing times before planning your itinerary to avoid disappointment.