How are people estimating LLM API costs before putting an AI application into production?

It's relatively easy to estimate the number of API requests, but I'm finding the token side more interesting because input and output usage can vary considerably between requests. Things get even more complicated when comparing different models or using long prompts and conversation history.

Do you normally build a spreadsheet for this, use the provider's pricing information directly, or use a dedicated calculator?

I'm especially interested in approaches that estimate monthly cost from expected requests, average input tokens, average output tokens, and the model being used.