Some of the sites we work on are insurance based. We save every step of the calculation (inputs and outputs, e.g. rates and sub-totals). An administrative user can see the entire calculation from start to end, including overrides at various steps (e.g. a manual discount), and breakdowns per state or item insured if appropriate. This seems like the acceptable bare minimum to me, rather than just showing a magic number. And it definitely helps to expose bugs.