I think the cost increases between armors make sense anyway. After all, you're increasing the strength of the hull more and more, but the mass of the ship appears to be staying the same (ie, cap8 doesn't make your engines work any less). That cap8 adds so much more to your hull while being as massive as cap1 (while being bigger, as per cargo size) says to me that it's either made with more exotic materials or more exotic fabrication processes (or likely both).