Septic System Replacement Cost. The cost of septic system replacement depends on the type of system installed. Here are the average ranges: 1  Gravity fed drainfields of all types: $5,000 to $10,000, or an average of $7,500.2  Mounds: $10,000 to $50,000, or an average of $30,000.nderstanding Septic System Repair Costs. 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.