NASA has proposed using the Space Launch System until 2050. Over $30 billion has been spent on getting the Space Launch System to its first flight. This might happen next year. In 2019, the White House Office of Management and Budget estimated the cost of one SLS launch a year at over $2 billion.
NASA proposes that they will be the main customer for SLS but pay half of the per launch price. This would be say paying $2 billion per year out of a $4 billion price. NASA has been paying $2-3 billion per year for the development of SLS with zero launches per year for over ten years.
There is not fixed price per launch yet.
SpaceX should attempt to fly the Super Heavy Starship to orbit by the end of 2021. SpaceX Super Heavy Starship will have more payload capacity than SLS and SpaceX SHS will be fully and rapidly reusable. The one-time cost of the SpaceX SHS will be $100-200 million initially but with frequent reuse has a target cost of $2 million per launch. NASA could end up overpaying for launch services by 1000 times.

Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
Known for identifying cutting edge technologies, he is currently a Co-Founder of a startup and fundraiser for high potential early-stage companies. He is the Head of Research for Allocations for deep technology investments and an Angel Investor at Space Angels.
A frequent speaker at corporations, he has been a TEDx speaker, a Singularity University speaker and guest at numerous interviews for radio and podcasts. He is open to public speaking and advising engagements.