Your BSPDC-1 provides 136 PPV for 433.6 BP, or 0.314 PPV/BP. Good, but I think we can do better.
PDD Dummy x24 1 class 1,500 tons 4 Crew 36.6 BP TCS 30 TH 0 EM 0
Armour 5-11 Sensors 1/0 Damage Control Rating 0 PPV 24
Intended Deployment Time: 0.0001 months Spare Berths 103
ICBM Silo (1) Missile Size 24 Rate of Fire 43200
This design is classed as a Planetary Defence Centre and can be pre-fabricated in 1 sections
ComponentsThis design provides 0.656 PPV/BP, or slightly more than twice as cost efficient. It also doesn't require any Tritanium or research in its construction. This means it is available at turn 0 even on Conventional start. I am not sure about the consequences of not having a Bridge, but it is buildable. The ~4 minute deployment time should prove entertaining too.
ICBM Silo x1
Conventional Armour x5.8
Crew Quarters - Tiny x1
I'm going to have to flag this one for being illegal on two fronts. First, it has no bridge and it's over 1000 tons. Second, it has no fire control. If it is buildable, that's only due to the way PDCs are built. Nice try, but not quite. And your other effort suffers the same problem. However, using ICBM silos is a good idea. A legal (lockable) 32-ICBM PDC built in the same game as my previous efforts is:
BSPDC-3 class Planetary Defence Centre 38,750 tons 108 Crew 883.9 BP TCS 775 TH 0 EM 0
Armour 5-102 Sensors 1/0 Damage Control Rating 0 PPV 768
Intended Deployment Time: 0.0001 months Spare Berths 107
ICBM Silo (32) Missile Size 24 Rate of Fire 43200
Hughes FPG-501 (1) Range 8.5m km Resolution 50
This design is classed as a Planetary Defence Centre and can be pre-fabricated in 16 sections
This comes in at .8688 PPV/BP, which is very nearly what you managed. (The total cost of my added components was 11 BP, which is negligible at this scale.) So I will concede that this is the best way to go about this. Although fitting it with a better fire control wouldn't be terribly expensive, and would result in a platform that is not totally ineffective if you fill it with outdated missiles. This does, of course, assume that the deployment time is possible. A quick check shows that going to .25 month (enough to make sure there aren't any problems) costs 40 BP, which is bad for efficiency, but still leaves it well ahead of my design.