1. A costing system comprising: a storage medium storing a costing model recording parent-child relationships between a plurality of stages of a production line, wherein each stage comprises at least two valuation types, and the valuation types comprise internal production and outsourced production, and the costing model records an internal cost and an outsourcing cost respectively corresponding to the internal production and the outsourced production for each stage; and a processor executing a costing engine to retrieve the valuation types corresponding to parent stages of a target stage of a production line from the costing model, generate a first group of valuation type combinations according to the retrieved valuation types, combine a target valuation type corresponding to the target stage with the first group of valuation type combinations corresponding to the parent stages to generate a cost identification, and generate a plurality of costing results for the target stage in each valuation type combination of the first group accordingly by retrieving outsourcing costs and internal costs corresponding to the respective parent stages from the costing model, retrieving an outsourcing cost or an internal cost corresponding to the target valuation type of the target stage from the costing model, and respectively combining the outsourcing costs and the internal costs corresponding to the respective parent stages with the outsourcing cost or the internal cost corresponding to the target valuation type of the target stage based on the respective valuation type combinations in the first group, thus to obtain the costing results for the target stage in each valuation type combination.