Hi @CY_Chang, thank you for posting. This documentation should answer your questions: Consumption model for Viva Insights | Microsoft Docs. Please let me know if you have anymore questions after reading this documentation.
I think this would be rather extreme if this above example used 3500 units given that 5000 is the base units pack. So it might be an omission in the first formula for the units. I'd encourage you to use this formula to calculate how much a one year of Standard Person query would cost. I guarantee you some emotional moments at the end. And these are not-so-good emotions, I am afraid 😞