These things are always hard to balance, there are no guidelines for it. Though your commodities choice and numbers seems fine to me, maybe they can be even little lower.
If you want some inspiration and data for comparsion, here or here are similar RP projects, just in large scale because they are about ship constructions.
Remember, though, that a defense platform is a self contained weapons system with 6 or more guns, aimpoint sensors, a computer and comms, possibly its own power supply, etc.