The increasing urbanization of urban terrain related to the development of underground infrastructure requires an effective way of draining and managing rainwater in urbanized areas. Designed cubic facilities, used mainly to accumulate excess rainwater and control its flow in the sewage system are very expensive. In addition, they require free land for their construction. A modern system, allowing temporary retention of rainwater in the sewerage is an innovative rainwater system equipped with damming baffles, which operates by gravity. Damming baffles have appropriately dimensioned openings in the lower part of the pipe in order to maximize the use of their capacity “up to the ceiling”. It allows the sewer system to function hydraulically favorably. The entire computational procedure involves simulation using hydrodynamic modeling, applying the author’s JDM17 algorithm to dimension the innovative sewage system. The purpose of the article is to present the development of investment costs of classic and innovative rainwater sewerage. Several design variants were evaluated financially, differentiating them by the geometry of the sewer. The dimensities of all pipes were deliberately increased. The analysis takes into account the actual conditions in the studied catchment area located in south-east Polan and the costs of constructing a sewage system for it, including the necessary materials. The studies carried out so far confirm the economic, hydraulic and operational advantages of the innovative rainwater system. It is a low-cost and modern solution, which to a large extent reduces the flow rate of rainwater, even more than twice. This also reduces the kinetic energy of flowing rainwater compared to commonly used classical drainage solutions.