Some of the most significant aspects in the selection of wastewater treatment plants are the investment costs, since they cross-linkthe treatment level, the quality of the raw wastewater, the design flow and the purpose of the treated wastewater. Through a multivariable exponential regression analysis, data from 51 projects of new treatment plants was analyzed, and from that process, data of cost scale elasticity was obtained, in slow growth, in comparison to the design flow for each of the treatment technologies analyzed.