A Business Case usually captures and motivates the reason why a given solution journey is embarked upon. Typically this is a well structured document that tells the story and outlines what benefit is expected at the end.
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This Post is not about the document but more about a way for the Solution Architect to quickly build a simple financial case in support. There is a fallacy that a business case is a thick tedious manuscript, written by professional consultants in an incomprehensible language. Hope it helps!!!