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Chatbot monthly cost estimator

Monthly cost of a chat assistant where every turn re-sends the conversation so far.

Model, priced from the vendor page on the verified date
monthly cost0.00

Prices read from the vendor pages on 2026-10-20. Arithmetic on list prices, not advice or a quote.

Pro

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How it works

Chat cost grows with the square of the turn count because each turn re-sends the whole history. Prompt caching turns most of that re-sent history into cache reads, which is why the cached and uncached figures differ so much on long conversations.

The formula

turn n input = system + n * user + (n - 1) * assistant; everything before the newest user message is cacheable; output = assistant tokens per turn

Worked example

With the inputs the page opens on:

  • conversations per month: 10000
  • turns per conversation: 6
  • user message tokens: 80 tokens
  • assistant message tokens: 250 tokens
  • system prompt tokens: 1200 tokens
  • model: claude-sonnet-5-5
  • cache history: true

gives

  • monthly cost: $171.75
  • cost per conversation: $0.0172
  • input tokens per conversation: 12,630 tokens
  • output tokens per conversation: 1,500 tokens

Sources

Prices read from each vendor's own pricing page on 2026-10-20; the page may have changed since. This is arithmetic on published list prices, not advice or a quote. Check the vendor's page before committing a budget.

Questions

What about summarizing old turns?
Trimming history caps the growth; model it by lowering turns_per_conversation to the window you keep.

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