A method of controlling an optimal cost is proposed, which can be applied to a circuitry designing process for making electronic products, allowing a user in drawing a circuitry to choose elements of identical specification with different unit prices, thereby achieving the objective of cost-optimization. The cost-optimization method comprises the steps of: first, integrating data of each of the elements of identical specification at different unit prices to form an element database; then, choosing each of the required elements for drawing the circuitry according to the data stored in the element database. The method provides an element database capable of storing and displaying electronic elements of identical specification with different unit prices for the users' selection, thereby achieving a primary objective of effectively controlling an optimal cost in circuitry designing.