One SpaceX Starship will cost about $5 million as SpaceX will mass-produce nearly one hundred in 2021. Each Starship and Super Heavy booster will cost less than about $40 million. Each SpaceX Super Heavy Starship will cost about $40 million. Before considering the full reusability of SpaceX Super Heavy Starship will cost one thousand times less than SLS. Full reusability will drive the cost down to $2 million or less for each SpaceX Super Heavy Starship launch. This means two thousand SpaceX Super Heavy Starship launches for the price of one SLS launch. Elon says SpaceX plans to build one Starship a week by the end of 2020. SpaceX is designing its factory to build one Starship every 3 days (72 hours). The three-day build target would be by the end of 2021.
SpaceX would make 120 Starships per year if they were building one four shifts for 7X24 production at the rate of one every 3 days. They would need to build at least 800 Raptor rocket engines for those Starships and likely would need 2000 Raptor engines if many of the Starship needed Super Heavy boosters with over 30 Raptor engines each.
SpaceX will be making a series of large tents and buildings to house its Starship factory.
Those tents and buildings will be the SpaceX Starship gigafactory. The manufacturing rate will be comparable to the speed of construction of commercial wide-body airplanes.
Elon has said that mass production will bring the cost of the Starship down to $5 million.