Gravity or Pressure

• A gravity septic is the most common type of system and the cheapest. A pressure system requires an electrical installation component and will typically cost more. You might expect to pay between $3,000 and $5,000 for a gravity system for a three-bedroom house, and between $5,500 and $8,500 for a pressure system, as of 2011.
  
  

Gravel or Vaults

• Your system can be drained with either gravel drain rock, or you can have plastic vaults installed in your tank. Gravel is a cheaper option, but usually requires a larger tank, so if you're tight for space, you may need to opt for vaults. This will put your cost estimate at the higher end of the range.
  
  

Variables

• The two biggest variables in the cost of your system will be the number of bedrooms in the house and the type of soil you have. The more bedrooms (and therefore people) the system must service, the larger the capacity required, so the cost will rise. The most expensive sites in which to install septic systems are areas with fine, silty soils, because more drainfield is needed.
  
  

Local Costs

• Labor and materials costs vary widely around the country, adding to the variability of your estimate. To get an accurate idea of the cost of a septic system, have your design approved under local planning laws, then take it to several contractors. Get at least three estimates before choosing.
  
  

Maintenance

• Septic systems are certainly not maintenance-free, and you should factor in the cost of having your system pumped every year or two. This may cost up to $150. Depending on the type of system you opt for, you may also find yourself having to replace pumps or other components from time to time, at prices up to about $500.