Estimate LLM cost

Cost surprises come from not doing the arithmetic. This skill ships a small, dependency-free calculator so you can price a call, project a monthly bill, and compare models with actual numbers.

Use the bundled script

scripts/llm_cost.py is pure Python, no install needed:

from llm_cost import estimate_cost, project_monthly

estimate_cost(1500, 300, model="gpt-4o")                       # one call, USD
project_monthly(1500, 300, calls_per_day=5000, model="gpt-4o") # monthly projection
estimate_cost(2000, 200, model="gpt-4o", cached_input_tokens=1800)  # with prompt caching

Run it directly to see a worked example: python scripts/llm_cost.py.

Prices in the PRICES table are approximate and change often, so pass input_price/output_price explicitly when you need exact figures, or edit the table. The arithmetic (not the price table) is what the tests pin down.

How to apply it

1. Price the call with realistic token counts (measure them from a trace, see instrument-llm-observability).
2. Project the bill with your real call volume. A cheap call at 10k/day beats an expensive one at 10/day.
3. Compare models by running the same tokens through two model ids. Pick the cheapest that still passes your evals (see compare-llm-models).
4. Feed it into cost cutting (see reduce-llm-cost).

Validation

Run the tests: pytest skills/estimate-llm-cost/tests/. They check the math is exact for explicit prices, that model lookups match the table, that prompt-cache discounting is correct, and that monthly projection scales linearly.

Anti-patterns

• Trusting the built-in price table as current (verify against provider pricing).
• Pricing one call and forgetting to multiply by real volume.
• Comparing models on price without checking quality holds on your eval set.