The point of the Devastator Cannon is to be, well, devastating. Though you can still devastate something if you happen to miss, it's more likely to be a space-orphanage. Thus the precision.
As to the gold cost, I already provided an explanation which was, admittedly, bad. I figured that since, as has been said, ships are going to be much more powerful than the average unit, they should cost much more than the average unit over time. Gems are often not much of a deterrent to summoning a horde of something; a huge upkeep is going to be. The Capital Ship has the highest protection, HOP, and damage of pretty much everything (short of Deadly Sign-equipped Great Old Ones, and it eclipses even Ktalu in pretty much everything) in the mod. It is meant to be a nigh-unstoppable super-weapon. It comes with a cost (two, actually, the huge gem cost and the huge gold cost). I don't want the Neoclidian Capital Ship to become a unit to summon in hordes and send in 10+ packs of 3 against enemies. I want the capital ship (every capital ship really, although I guess it depends on whoever makes the capital ship) to be a single unit that could take on small armies solo and, combined with the many units it can lead, become an unstoppable fleet hellbent on the awakening of the Great Old Ones. I noticed that UlmInSpace's capital ship was recruitable (and, well, just a recruitable ship), and didn't want that to happen to the Neoclidian fleet.
On a semi-related note, I may add recuperation to the Capital Ship to represent nanites and such, and also because a Capital Ship that's lost its eyes or one of its arms isn't much of a capital ship at all.