Valuation of a septic drain field?

[Jim L.]

Landlocked property has an easement over a 1/4-acre ell in the neighboring property that otherwise blocks the property from the road and is largely (desirably, for privacy/noise) covered in trees.  Owner of neighboring property is willing to selling the ell, and develop-able vacant land in the area goes for $10-$15/sf normally.  Some larger (8+ acre), treed, hilly land nearby seen for $2-$3/sf.  However, the ell includes the neighbor's septic drain field.  Sale of the ell would require an easement for the septic drain field.  So...  as the neighbor would retain some value in the land (easement for drain field) and the land is essentially un-developable, what is a fair price range?