1 Mound System-A mound septic system cost will depend largely on the slope and grade of the property, and these systems can cost from $9,000 to $25,000 or more.2  This type of system can have many variables which can affect the cost you will pay. Wisconsin At Grade System-This type of system may run from $6,000 to $10,000, and involves a mound. 2  A Wisconsin at grade system is rare, and is not used often. 3  Evapotranspiration System-The septic system cost for this system type is high, and will start at about $25,000 and go up from there.