I once quizzed August on this very thing.. apparently there is no standard rules by which vessels are constructed they are simply built on a case-by-case basis.
I have even tried collating all vessel info trying to find some common denominators by which to determine a system and in all honesty I couldnt find one. I suspect any coherent system, made to build ships for Bab 5 would be a prelude to a heap of revamps and re-writes concerning ships released so far in order to make them usable by whatever system was made.
At this point your probably thinking.."um well thats no good", but in all honesty just making a vessel to suit your needs at the time and statistically giving it what bonuses, modifiers and features you need really does work as good as any system.
There was a system done for modifying vessels, allowing you to customise various aspects of them but that was in the Babylon 5 Companion that has yet to see release.
Honestly I don't see much need for a "construction system" - just model them according to closest matches with existing ships (Ship X needs to have that weapon from ship A, that from ship B, is as big as ship C, and needs to have a speed partway between ships D and E).
But then what do you do when your players want to upgrade a ship to have new weapons (or even give it weapons in the first place).
My players own an old Centauri Supply ship and have asked about fitting a gun or two to it, but without some idea how to allow that, costs, and keeping it balanced, I've had to ask them to pospone that idea for the moment.
I'd really like to see some construction and modification rules.
Modification rules would be good - especially guidelines for taking that civilian freighter and welding on some armament
I submitted some to S&P as part of another article, so its possible they might see light of day.
Costs for construction from scratch - building a new ship as a one-off would have extremely prohibitive costs - you have to get a shipyard to tool up just to build your custom hull. It'd be like asking a plastics company to run off a single sprue of something. Unless you own the shipyard in question the chances are its not gonna happen, so its something best played through for those characters who own companies that can afford to do such a thing, without it hitting their personal wealth. And then you can simply model the stats as GM to be close to what they've asked for. The process itself ought to be fairly abstract - players shouldnt be able to ask for a point of this or that, just "Make the hull stronger than the X class, make it faster than the Y" - it really ought to be RPed between them and the designers/engineers they're employing to do the task.
I would also like to have some rules for refitting/upgrading ships. I started to make some rules for that for the old B5 (Camelon Eclectic (sp)), but never got satisfied with it so I never finished it. I wanted to make some rules that would govern fighters and up to medium sized freighters, since that's the most likely shiptype that characters would own.