I think business cases are important but it is not necessary or even beneficial to be overly precise and granular in the quantitative sections except as form of organizational signaling (look how thorough I am). If you compared the pro forma’s I did as a 28 year old with the ones I do today you would have seen a lot more rigor back then. That rigor was a complete waste of time.
Last year I did a strategic engagement for a large multinational. The whole experience was shall we say? Less than rewarding. When things go wrong you must ultimately blame yourself for one simple reason, if you are responsible for all the good that happens to you then you are responsible for all the bad.
Part of this engagement was to develop a business case which I never quite got to, due to all the time wasted in hand wringing over slides on a PowerPoint (which seemed to be the required mode of management communication within the company) and waiting for feedback. Regardless, I was handed a template for a business case (which looked like it had been developed for manufacturing not IT) and it looked like it was written by a finance nerd in an MBA program. I did find out who wrote it for them (a large multi-national consulting firm) and the firm was apparently being paid by the bit. It had an extraordinary amount of granularity and to complete it would require a team to collect all the inputs. In short it is a complete waste of time.
Why would I say that? Because bottom up detail does not equal forecast accuracy. When you develop a financial model based solely on internal estimates it’s going to be way off. The only way to get any degree of accuracy would be to have your CIO call three other CIOs at companies whose size and complexity is a rough match and ask them how long it took them to deploy and how much did it cost. That beats the vast majority of internal estimates. The hardest hurdle to get over is the IT’s confidence in their ability to do a better forecast. Will they? Maybe. Statistics indicate they will not.
If the outside option is not available then use your company’s own experiences in IT projects of equal complexity. You will save a lot of time and effort and perhaps improve your accuracy.