It will cost tens of billions to develop these weapons for the USA, China, Russia and other nations.
However, the USA will be gifted complete space dominance via Spacex fully reusable rockets. The US already has superior space capabilities versus other countries.
In Elon Musk's video announcement of the Spacex BFR he indicated that it would be lower cost to launch than the Spacex Falcon 1. A graphic showed the Spacex BFR at lower cost than the Falcon 1.
In 2005 Falcon 1 was advertised as costing $5.9 million ($7.3 million when adjusted for inflation in 2015). In 2006 until 2007 the quoted price of the rocket when operational was $6.7 million. In late 2009 SpaceX announced new prices for the Falcon 1 and 1e at $7 million and $8.5 million respectively, with small discounts available for multi-launch contracts.