average septic tank cleaning cost average septic tank cleaning cost is $ 200 to $ 300 for a septic tank as large as 1000 gallons prices can go up to $ 800 for a large 2500 gallon tankthere may be an additional $ 200 to $ 400 charge if the contractor has to find the septic tank and locate the opening depending on the amount of labor involvedarbage disposals increase this schedule by about 20 % as well septic system additives that help control bacteria can help a little bit but it will not replace pumping out your septic system average septic tank cleaning cost is $ 200 to $ 300 for a septic tank as large as 1000 gallons