A cost comparing system includes a plurality of client computers (10), an application server (12), and a database (14). The cost comparing system is connected to a manufacturing system (15), a supplier goods providing system (16), and a sales quoting system (17). Each client computer provides a user interface for inputting and obtaining information from the application server. The application server is for computing item costs and comparing costs with prices. The application server includes a basic information maintaining module (21), a BOM copying module (22), an outsourcing item cost posting module (23), a standard BOM computing module (24), an item cost collecting module (25), a price maintaining module (26), a comparing module (27), and a statement generating module (28). The database is for storing basic information, computing results, maintaining results, comparing results and generating statements. A related cost comparing method is also provided.