Billing and Pricing: Cost Drivers and Optimization Levers
What to know
Billing questions test whether you can identify the dominant cost driver and choose the right purchasing lever. Understand the main pricing dimensions: compute billed per second or hour, storage per gigabyte-month with tiering, data transfer out of the network (often the surprise), and request or operation counts for serverless and APIs. Inbound data transfer is commonly free while outbound is charged; inter-region traffic also costs.
Know the purchasing options. On-demand gives flexibility at the highest unit price. Reserved or committed-use discounts trade a one- or multi-year commitment for a lower rate and suit steady baselines. Spot or preemptible capacity is cheapest but can be reclaimed, so it fits fault-tolerant batch work. Savings plans offer commitment-based discounts across families of services. Understand budgets, alerts, tagging for cost allocation, and consolidated billing across accounts.
Traps
The most common trap is optimizing the wrong line item - shrinking compute when egress or idle storage dominates the bill. Another is recommending reserved commitments for unpredictable or short-lived workloads, which can cost more than on-demand if unused. Watch for questions implying that stopping an instance eliminates all charges; attached storage and reserved addresses may still bill. Also distinguish a budget alert (notification) from a hard quota (enforcement).
Worked rule of thumb
Estimate monthly cost as the sum of each priced dimension, then apply the lever per component: steady baseline to commitments, spiky or interruptible load to spot, and one-off or unknown duration to on-demand. If an answer applies one lever to the entire estate, suspect it.
Study approach
For each scenario, ask which dimension dominates and whether demand is steady, spiky, or interruptible.