Why do PCBA processing plant project charge cost

In the price of PCBA processing, mainly contains a circuit board production costs, components procurement costs, assembly, engineering, logistics, customs clearance service and follow-up, etc., please, some of them for charge cost is not very understand, why to talk about the following printed circuit board assembly project charge cost. Generally in the mass production, will conduct first article proofing, while others, such as production process documents, stencil making, SMT line change, these are need to customers should bear the cost of, this is the construction cost. Construction cost generally applicable to small batch orders.