[AccessD] Access Application - per unit cost

I have been working on an Access application for several years.  My client has asked for a "per unit cost" for the development of this application, although he is unsure what "unit" is most meaningful (e.g., database object, lines of code, etc.).

The database functions vary tremendously in their complexity --- e.g., some reports are very simple while others contain several subreports, and/or output spreadsheets or files.  The database contains lots of VBA code.

Are there any white papers, etc. that address this issue?  In your opinion, what is the best method of computing a "per unit cost"?

Thanks,
Barb Ryan