I personally don't like high resource costs, as it doesn't really fix the problem. Stack up steel to your maximum thresholds, then build PD. Add most existing PD great-walls-of-china have been sitting around for a while, and so probably have big piles of resources already accumulated. IMHO, the most effective means of balancing PD is not to raise the cost of PD, but to diminish its power.
And once that resource pileup is used to build a fleet and they see they're only making 2000 steel per turn instead of 12 000, they'll seriously consider whether they need all those planetary defenses. They'll certainly be worse off than someone who has a balanced setup since a single non-pd planet equals six that have one
