That's not how a launch cost is calculated. The cost of the rocket's construction is included in every launch, hence the cost going down with subsequent launches. RND costs must also be included, as well as support system costs, staff costs, etc. Also, current starship designs require refurbishment between launches, as well as an expensive set of checks and tests on systems. 1 billion as the cost for the first fully test launch is not at all out of the ballpark.