1 The average septic system replacement cost ranges from $6,000 to $50,000 including financing. 2  Up to 50% of the replacement septic system cost is profit for the installer, therefore our solution is rarely mentioned.3  Most regulators are risk averse and avoid breakthrough technology. The average septic system replacement cost ranges from $6,000 to $50,000 including financing. 2  Up to 50% of the replacement septic system cost is profit for the installer, therefore our solution is rarely mentioned.