There are many variables which need to be known to properly answer this question. The first is what part of the country you live in? The cost probably varies from $2000 to $20,000 depending on soil conditions, local regulations, and type of system to be installed. Systems are generally less costly in the southeast and south and more expensive in the northeast and mid-west.

If the local regulations require an engineered system, the cost will be higher. If the soil has poor drainage, the cost will be higher. 

The best advice I can offer is 1) Do not skimp of the soil testing, 2) Build the system large enough to easily handle the largest family you might have, and 3) Ensure that there is either a distribution box or a clean out, riser, between the septic tank and the drainfield so that the drainfield can be monitored and treated if necessary in the future.

After the sytem is installed, proper maintenance and pumping will help ensure many years of operation.