Because there's no loophole to exploit here, the exchange prices are clearly calculated.
However, among the building blueprints that can be exchanged, Zong Shen will only consider exchanging for active defense buildings like defense towers, and will not exchange for things like city walls or road barriers.
Since defense towers are all one-time blueprints, normally one set of defense tower blueprints can only construct one building, with many restrictions, whereas things like city walls and road barriers can be repeatedly constructed as long as one has mastered the blueprints, with some resources and time spent.
Another point is the cost issue; the blueprint to exchange for one section of Tier One City Wall requires 22 points, and the material cost is 20 points, with a premium of 2 points since a complete blueprint can be directly placed without labor and with a very short construction time, so the premium of 2 points is quite normal.
