Why is isolating the precise cost dedicated only to the physical construction of ISS modules nearly impossible?

Answer

The total expense incorporates design, logistics, research grants, and management overhead

The complexity of the undertaking means the total expense inextricably includes elements beyond just the physical hardware, such as research grants, logistics, management overhead, and the cost of enabling infrastructure necessary for assembly.

Why is isolating the precise cost dedicated only to the physical construction of ISS modules nearly impossible?
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