An NPC would collect necessary items like crystals or whatever random crap you think needs to be sunk from the economy. Each item would be worth some number of "construction points" depending on its value, and when enough total points have been contributed, the new ward is built. For an arbitrary example, this could start at 1,000,000 for the first constructed ward, and increase by 3% for each subsequent ward.
(If, somehow, this runs up against actual server limitations, the NPC could say something like "well we're ready to construct and just waiting on the permits" until the server limitation was handled and the new ward could exist.)

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