I just had a quick look at the specs. (18-105mm, 16-85mm). They have different construction:-
18-105mm: 15 elements/11 groups, 1 x ED element, 1 x aspherical element
16-85mm: 17 elements/11 groups, 2 x ED elements, 3 x aspherical elements
I assume that ED and aspherical elements are more expensive to manufacture than standard elements.

The 16-85mm should theoretically perform better than the 18-105mm with regard to issues like chromatic aberration, however I don't know how either lens performs in practice.



Cheers.