The question of Return On Investment (I really try to avoid jargon - yeah, even in this business!) is, as far as I'm concerned, one of the 3 reasons I do "ISO consulting".
First reason - make a living and something I like to do.
Second reason - provide my client with a better way to do things.
Third reason - make it worth his investment!
Analysing processes from the payback perspective can be done as a separate endeavor - "cycle time management". Or it can be done in conjunction with the discipline that already analyses processes (from a conformance perspective). Or you can do the client a favor, if they agree fron the start, and do both.
Yes, it is harder. Yes, it is not traditionally "part of my job". No, I don't give it away. But if you want to make a good living and really give a client significant benefit from the disruption in their business that will go through anyway during implementation, advise them that instead of just doing a "cosmetic renovation", you can also show them which "machinery" could be changed to improve things.
How is the value of a new QMS calculated? CSI, when all the smoke and mirrors are swept away, doesn't give any better answer than a wide-area weather report. You have to wrap time and cost around those processes that are to be measured.
My present company wrote a book on that, called "Cycle Time Management". Even that book can't provide enough info on the specifics of how to calculate the value of a QMS, so I certainly won't bore y'all with it here. But it can be done!