At tech 12, you pay MCr 4430 to move 4462 dtons three parsecs at a time - roughly MCr1 per dton of cargo capacity.

At tech 15, you pay MCr 2989 to move 5067 dtons three parsecs at a time - roughly MCr0.6 per dton of cargo capacity.

TL15 is therefore roughly 50% more efficient at moving cargo than TL12.

However, lets look at military starships - lets look at the minimum spend to shoehorn jump 3, 3G and 3 armor into a ship that mounts a type A Spinal PAW.

OK, call it roughly GCr23 at TL12.

This is a ship with the same specs at TL15 (and Im aware a very small investment in a better computer - which in HG presumably includes the sensor package - will make a much better warship).

Call it roughly GCr10.5 at TL15.

At TL15, military ships of the same specs are at least twice as effective than at TL12.

Therefore, TL12 shipyards should build lower tech but good enough cargo ships, selling for enough Imperial Credits to buy the high-tech warships that TL15 shipyards will specialise in.

In the Spinward Marches, that means Glisten and Rhylanor would build most warships, while civilian craft are more likely to come from lower-tech worlds like Regina, or are exported from the Sword Worlds.