A cost calculation needs a quantity, unit, time window, and rate for every line. Mixing monthly storage quantities with hourly compute quantities without tracking units produces a plausible-looking but wrong total. Fixed and variable components also behave differently as usage changes.
Every numerical rate in this course is a fictional exercise input, clearly labeled as an example. It isn't a claim about AWS pricing, discounts, taxes, or regional availability. For a real estimate, obtain current vendor rates and the workload's measured usage. In these exercises, include every supplied line, check the units, and keep a zero-use line at zero. A lower unit rate doesn't prove a lower bill if the design increases the amount consumed.