Cost-Benefits of HF in UI design

You might like to look up the work of Claire Marie Karat.  It might give
you some cost factors to look at.  Here are a couple of WWW references to
some of her published work on the "Cost-Benefit Analysis of Iterative
Usability Testing":

>From HFS '90:
   http://www.tu-graz.ac.at/0x811b0205_0x0005dae1
>From Interact '90:
   http://www.tu-graz.ac.at/0x811b0205_0x000ced66 

Both references wer found from TUGinfo whose home page is
found at http://www.tu-graz.ac.at/

Bias, R. G., and Mayhew, D. J. (Eds.), 
Cost-Justifying Usability. Academic Press, Boston