A supercarrier is not a spaceship. Nor is it build like a spaceship.
Human propulsion technology, right now, sucks. It's expensive and inefficient. Not suitable for sci fi at all. Space battles CANNOT exist with modern technology, as proven by the space shuttle comparison.
There is no reason to believe that reactors, guns, or engines would have anything other than the same cost per ton. I don't see any complexity issue that makes them more expensive to provide the exact same efficiency at a smaller size. Perhaps you meant to post an example but didn't.
Empty space adds volume, and volume full of mostly air reduces the density, which has an effect on "price per ton" as you say it, but also "price per unit of volume." Displacement is a stupid term in space, as there's no water to displace... only volume and mass really matter.
No one has ever made a large ship in space, as far as I know. So yeah, the space shuttle sucks as a comparison. You can't compare spaceships to ocean ships... major fallacy, there. A rocket engine is more expensive than a turbine. Duh.
As for the turret vs. big guns analysis...
So your turret fires a 100 mJ particle gun at a battleship. Battleship's armor can completely ignore anything short of 1000 mJ completely. You fire 20 of those 100 mJ particle beams... battleship armor reduces the damage by 1000 per shot. No damage taken.
Meanwhile, I fire a 2,000 mJ spinal mounted particle beam at battleship. Battleship armor reduced 1,000 of that energy. Battleship is destroyed by the other 1,000. Win for me. Big guns are better for taking on armored vessels. At the range at which space battles take place, it doesn't MATTER if your weapon is only pointed in one direction... it's not like you can dodge it any easier than you can a turret. As far as the ammo problem... just use energy weapons, then. Don't require explosive ammo. Problem solved.
And Traveler is.. well, a lot less soft than a Dr Who, and a lot of other things I've seen. Soft vs hard is a sliding scale, you can't classify one or the other. If you could, there would be no difference between Star Wars and Starship Troopers (the book was better than the movie).
A little numeric experiment with spheres and turrets. Let's say you have a 450 foot diameter sphere. Using 4/3 pi r^3, you can determine the volume to be roughly 36 million cubic feet. And using 4 pi r^2, you can calculate the volume as 636,000 square feet.
Now say you put all of the weapons on external turrets. Turrets need room to maneuver... a lot of it. So roughly 20% of your sphere's surface can be covered in turrets, or 160,000 square feet. Now you have an option between small point defense turrets (400 square feet each) and large weapons platforms (5000 square feet each). If you use entirely large weapons playforms, you can fit roughly 32 turrets on the ship, each turret supporting some 20,000 cubic feet worth of guns. And that's close to the maximum firepower of a ship that size's turret array.
Meanwhile, I alter the shape slightly, increasing surface area to 660k square feet... not a huge increase. I use nearly all of that surface space for external point defense turrets, for a whopping 325 PD turrets. And then I mount... get this... 6 MILLION cubic feet (out of 36 million) worth of spinal weaponry. So while your ship has 640,000 ft^3 worth of externally-mounted guns, my ship has 6 million ft^3 worth of spinal-mounted weaponry. It's pretty clear to see which can bring more firepower to bear on its target, and at long range... it really doesn't matter if you can only point your guns in one direction. You're fighting at thousands of miles, minimum. Probably not light seconds due to speed of light lag but... whatever range you can reasonably expect your shots to hit.