Seeing there's so much construction spam on siege servers thesedays I propose that costs are added per deployment to the following items:
Construction Site
Construction Material
Siege Shields
Ladders
Meaning that for example, a ladder has a fixed cost of 50gold, using 3 ladders in a round means you then pay 150gold extra upkeep at the end. Obviously the exact charges would need to be balanced so as to not completely discourage their use but also prevent insane spam.