I do recall that both NPCs and AI would be available to turrets so they will be capable when not manned, but that PC manned turrets would be designed to always be more effective.
My guess is if all you want is to make the turret functional, the NPC option will be more expensive over time and the AI option will be more expensive up front. Either way, PCs will probably cost more and be more effective, with the exception that PCs will sign on to ships like the Javeline and Bengal for free inside an org, just because they want that kind of group experience.
I think what player shwo buy very expensive ships but don;'t network and make friends wilol find is that it costs much more to man a Javeline than it does to purchase it, and as a consequence many of them will just sit.