We used Ames Associates in Meredith to design our septic system in 2004. They did an excellent job, both in designing the system to meet all applicable regulations and in obtaining the necessary construction permits. Unfortunately I don't remember the cost, but they could probably give you a ballpark estimate over the phone.
Here's a link to their website:
http://amesassociates.com/septic_sys...front_ames.php