In GalCiv 3, you built dozens of constructors to max out all your starbases. You could request a constructor from the starbase screen that would either summon one already built or cause one to be built but, that was still ship production and time to move to the base. Additionally, you need to have prerequisite tech research to upgrade a starbase. You'd need starbase defenses to build that module, regardless of if you have a constructor.
In GalCiv 4: Supernova, there is a push to make Starbases more vital and more substantial to the early game. Costing a population reflects the ability to extend your ship range to new systems, your empire borders, and your ability to land grab systems. With the REALLY cheap draft colonists project, you still can send out colonists to setup your colonies and core worlds and you don't need to give up your pop. The module/module pool system is a pretty nice way to upgrade your starbases quickly, efficiently, and safely.
I'm interested in seeing how everything will shake out in the end.