With modern day computers the cost of designing a new ship is pretty much entirely just money and time. This is admittedly in no small part because there are massive databases with known materials and their behaviours that can be referred to when designing and simulating a new ship, obviating much of the need for prototypes and small scale tests that are later scrapped.
Retooling = Ordering and installation of tools needed to make high end ( military or aerospace grade ) products today is a huge part in the cost and time delays of anything you want to make that's larger than a cellphone...
Probably even bigger than it was in the old times when sub mm precision was not needed, and when your tools were not manufactured on the other side of the globe by some specialists...
If you want a more accurate or realistic cost of retooling and shipyard flexibility what you could do is the following:
- Each generic ship component gets a "re-usability" stat value which represents how much of previous ship designs can be reused.
- All player designed ship components have say 95% re-usability value ( As long as you re-use the same component retooling is super cheap )
- The cost of retooling from one ship design to the next then is calculated by using the ( 1 - [re-usability %] )
Example:
You swap from a ship using 250 ton of crew quarters that have a re-usability value of say 70%, to a new design that need 450 tons of crew quarters. Since you already have the tooling required to make 70% of what's needed you only need to pay for 30% of those 450 tons of crew quarters retooling impact. This is then applied for every component in the ship, until the final cost of retooling or a true average % is arrived at.