I could always make armor 10x as expensive as it is now to build... which I plan on doing anyways.
What about the Shield Arrays? They make at most 20000 shield points, and cost less than half of a Massive Planetary Shield Generator to build.
What sort of shield values are you thinking of?
Keep in mind that if all your planet has on it is armor WPs, with no weapons, troop ships will waltz right up and drop troops.
Might want to have at least one weapon on each.