The person recommending the technology should not be the only one valuing it
The person recommending the technology should not be the only person estimating its value.
This sounds obvious and it is violated in almost every approval meeting. The team that wants the tool writes the business case. The vendor that sells it provides the ROI model. The consultancy that will implement it sizes the benefit. Each of them is competent and each of them is grading their own homework.
The pattern is easy to see in the numbers. Benefits are projected in year one. Costs are projected for the build and stop there. Adoption is assumed at 100%. The "do nothing" option is described as catastrophic in one sentence with no figure attached.
Nobody is lying. Advocates believe. That is what makes them good at their jobs and useless as estimators.
A second estimate does not need to be sophisticated. Ask someone who does not benefit from a yes to take the same proposal and produce their own version of three numbers: total cost including three years of running it, benefit with realistic adoption, and what the cheapest alternative would deliver.
If the two estimates are close, approve with confidence. If they are far apart, the gap is the conversation, and it is a far better one to have now than in the post-mortem.
The advocate's estimate tells you what they hope. The independent one tells you what you are buying.