I've actually wondered if breaking overlapping PD is really even possible.
With, say, 7 or 8 PD overlapping on planets that are quite productive and that are close together (a star cluster), and with those planets able to produce 15,000 Steel a day each...
Yeah.... I feel like that'd be pretty tough to break. How many thousands of ships would you need?
Or you could just say that PD only has a limited amount of "shots" per turn. Overlapping would still be valuable, but less so.
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.
