Cost of sustaining N requests per second at your token sizes.
Requests per second is usually discussed for rate limits, but it's also a cost rate. Convert your traffic to a monthly bill and there are no surprises. See limits on the rate limit calculator.
Free concurrent-requests cost calculator — requests per second and token sizes to hourly, daily and monthly API cost.
Cost per request × requests per second × seconds. For a monthly figure at steady load, multiply by 86,400 (seconds/day) × 30. It answers 'what does this throughput cost to run 24/7?'
Peak RPS is a worst-case ceiling; average load is what you actually pay. Size infrastructure for peak but budget dollars closer to average, since traffic is rarely flat-out all day.