1 The cost of a septic tank alone runs about $500-$1,800 depending on size (ranging from 300-1,000 gallons) and type. 2  Piping and other needed items adds another $100-$200 to the total cost of materials.3  New Mexico State University provides an introduction to installing a septic tank.iscounts: 1  Most counties and states require a building permit for installing or replacing a septic system, at a cost of $250-$1,000 or more depending on the location and the complexity of the project.