I'd personally opt to leave their build & maintenance Cost the same, but make them a metric crapton more tanky + Upgrade their Platforms to be 2L for the Large Platform Type, 4M for the Medium Platform Type, & 6S for the Small Platform Type... after all, they have 0 Propulsion Systems vying with the Weapons Systems for CPU Attention or Power Needs...
Also I sorely wish we could choose how we Armament our Starbase Weapons themselves... Defense Platforms would be more useful if it was actually POSSIBLE to have our Starbase Armament & Defense Platform Armament synergize with each other instead of non-synergize.
Ya'all's mileage on this idea may vary though, no sweat. I personally can't help but to prefer Might & Brawn & Quality, & not so fond of cheap wuss spam.