Valuation of copyright in drawings and equipment manuals

A correspondent has written to ask the following:

"I have a question relating to valuation of copyrights in drawings and instruction manuals of plant and equipment (e.g. machinery).

Is the value of the copyright separately identifiable from the plant and equipment or is the value inherent in the value of plant and equipment?

If it is separately identifiable, then how does one value the copyright if there is no market for these drawings and instruction manuals?"

I suspect that readers of this weblog may have some useful guidance for our correspondent. Please post your comments below, if possible, of email them to me here.