Any reason why at least the ability of unlocking what is in effect rebuildable Mk. IV advanced starship constructor (a mark II mobile repairer/shipbuilder under this new system), but leaving their net knowledge costs to get that thing be at the 15000 knowledge? Besides some people complaining about it.
There are already some very niche, almost gimmick like options in the game due to their sheer cost (Mk. IV Scout Starships, Mk. IV and Mk. V ions, Mk. IV beam turret, Mk. III nuke, the current Mk. IV enclave), despite them doing their jobs well if you do get them, in the vast, VAST majority of situations, are usually not worth the costs.
That's fine to me, as long as you make it clear it is supposed to be as such in game, so people don't complain about it, or at least as much. And if they do, if it is clear in game, you can point them right there and say "the game warned told you it almost always isn't worth it, and they are designed to be as such, why are you complaining that it is?" (or something along those lines).
IMO, gimmick options are fine (and can have some actual usefullness in strange situations) as long as you make it clear it is a gimmick option.
I'd say leave in something that can be a rebuildable, mobile Mk. IV factory with the proper knowledge costs, but make sure to include a "this is a gimmick option" like blurb similar to what the Mk. III nukes and the Mk. IV and V ion canons have in their descriptions.