Hi Ovid, thanks for the brainfood you write that I get to read during my lunch break...
but... am I just off the planet in this - why hasn't someone mentioned Function Point Counting?
It's an industry-recognised metrics technique. Of course it may be that this is a very large sledgehammer for a small nut... but there may be a kernel (sorry) of an approach for you. And yes, this is your client's problem as dws pointed out.
Disclaimer: not a member, nor a knowledgeable FP Counter 8-)