I found this white paper about putting together a business case on IT Toolbox. It depends on how you interpret the term ‘business case’ but for me this document is more a feasibility study: what is the problem, what could we do about it and what have we decided to do. A ‘pure’ business case already knows the solution and is putting forward business and financial justification for how you are going to implement it to address the specific business problem. I would include the actual expenditure that will be incurred by the solution, resource costs, the reasons why other solutions were rejected and relevant financial measures like return on investment, internal rate of return and so on (although sometimes you’ll find a project has such a short timescale that time-based financial measures offer no insight for decision making).
There needs to be a strong justification as to why this solution has been chosen: sometimes, for example in the case of a change in the law, there is no choice but to implement the required steps to become compliant and any resource or other costs will just be written off as ‘cost of staying in business’.
My different interpretation of what ‘business case’ means might not be the same as yours or the same as the white paper’s authors, but it does highlight the reason why it is important to understand what you are producing and why. Who is the audience for this document? What will they do with it? Is it for information or approval? And how do the answers to those questions change what you are writing?