One of the significant aspects in the selection of conventional treatment plants for drinking water, are the investment costs, due to the relationship among the plant capacity, the compliance and the technology to be applied depending on the type of raw water. By means of an exponential regression analysis, in this paper we use data of 44 projects of conventional new treatment plants, where a history of elasticity cost of scale were obtained slow growth compared to the design flow for each of the plants analyzed were used.