Several factors can affect the cost of septic tank cleaning in Johnston. Dumping out septic tank can cost you around $70 to $200 and can run as high as $400 or more. It should be done every one to three years depending on the size of the septic tank and how many people are using it. Pumping out much larger septic tanks usually costs around $200 to $400 or more. The capacity of tanks like these is around 1,500 to 2,500 gallons. If you want to install a filter to protect both your leachfield and drainfield, you can pay around $200 to $300.