What does one AI action cost?

-- cost_per_action.sql (PostgreSQL)
-- From the talk "Your AI feature is an architecture problem" (netninjas.eu/events).
-- Model cost per customer and per AI action over the last 30 days.
--
-- ai_requests:  one row per model call (tenant_id, action_id, model,
--               input_tokens, cached_input_tokens, output_tokens, created_at).
--               input_tokens counts the tokens billed at the full input price.
-- model_prices: one row per model, in euros per million tokens, copied from
--               the provider's current price list.

WITH calls AS (
  SELECT
    r.tenant_id,
    r.action_id,
    ( r.input_tokens        * p.input_per_million
    + r.cached_input_tokens * p.cached_input_per_million
    + r.output_tokens       * p.output_per_million ) / 1000000.0 AS cost_eur
  FROM ai_requests r
  JOIN model_prices p ON p.model = r.model
  WHERE r.created_at >= now() - interval '30 days'
)
SELECT
  tenant_id,
  count(DISTINCT action_id)                                AS actions,
  count(*)                                                 AS model_calls,
  round(sum(cost_eur), 2)                                  AS model_cost_eur,
  round(sum(cost_eur) / count(DISTINCT action_id), 4)      AS cost_per_action_eur
FROM calls
GROUP BY tenant_id
ORDER BY model_cost_eur DESC;