The Nebula is nearly non-existent because of the Bantam books being largely ignored by the rest of everyone, so I keep hearing, rather than it being rare for any realistic sense.
The cost issue is basically the same as with the Lancer, as I brought up. It's much cheaper for anti-starfighter work compared to other anti-starfighter ships, and even does it much better. I think it's something to do with basically taking the success of what you have (Imperial-class), and just optimizing it, rather than coming up with something new.
I still have somewhat of a problem with the Republic being a general purpose cruiser, and the Nebula being the heavy hitter, because the Nebula would still be able to fill the Republic's role at a reduced cost.
It's probably because the Nebula's complement is 60 starfighters/shuttles. If that's reduced, it not only fits the smaller ship design, but allows the Republic to take on the role as a general purpose cruiser, having the larger starfighter complement. But as is, the Nebula has more/stronger weapons, stronger shields/hull, as well as a larger complement, all at that reduced cost.
By looking at the ship designs though, it almost appears as if the Nebula is more of a "get in your face" kind of ship, designed for broadsides with targets all around it, whereas the Republic is designed off of the principle of the Imperial, where the ship design funnels all weapons into a forward arc. This does make sense, since the Nebula's shields are supposed to be so hard to crack. Dive in and do massive damage, and come out unscathed.
You know, I really think that's a great analysis. The Nebula has such a dimunitive size - it doesn't make sense that it have such a large starfighter capacity compared to other ships. When canon specs conflict, i think reason should rule, which is what you've done here. It's easy enough to explain that even though the same number of weapons exist between both SD's (roughly), the Neb's are more modern/powerful. Not a great argument, but believable enough to me. Giving the Neb hardy shields and a powerful armament definitely makes it more of a central vessel that can soak up damage/tank, while being in the middle of a firefight; also, it's small size makes it more likely to be able to dive into a fight and back out with the least amount of nit-picky manuvering. Enter the Rebs as a general purpose cruiser (GPC), similar to how you'd use Escort Frigates or Dreadnaughts in earlier battles. And as you mentioned, as a GPC it will still be valuable down the road, as older more lightly armed escorts fall by the wayside. The Reb could have weaker shields and armor, but a decent armament and ability to carry fighters as well.
I do think there is an opportunity for the Neb to stand out even more given it's missile complement via Assault Conc. Missiles. Making it a dedicated anti-starship vessel nicely differentiates it from the Reb, while also giving it a specific and welcomed role in the NR navy. Having a stripped-down warship built for capship on capship combat isn't something the NR fleet really has, given the dual use of MonCals in the timeline as carrier/destroyers. That being said, I suppose the Neb should have a weakness highlighted; lack of fighters of its own anti-starfighter weapons, so it requires escorts? Given the game mechanics, I feel like that still doesn't bring the reasonable cost of the Neb down to where the Reb is, but moving in the right direction...
The inconsistency of canon ship specifications is annoying. They tend to introduce great ships in EU, but not really provide much information. And then when information is finally provided, it is conflicting compared to other ships in the timeline. The Black Fleet Crisis books really did muck things up a bit. That being said, I've always loved the Nebula. Given the importance of fighters in EaW/PR, the lack of fighters complementing the Neb may be significant. But the added Conc launchers should boost the price... There really isn't much information regarding the shielding of the Neb, but we both agree it should be strong. The only real motivator for AI/human players would be to get the best ship for the lowest price; if the Reb can still fulfil the GPC/carrier role even when the Neb comes about, then I think players will still build it.