Cost metric definitions that survive audit
Updated: September 27, 2026
Ask three teams for cost per request and you will get three numbers. That is not a rounding disagreement; it is three different denominators, none of them written down. In a board pack the metric looks like a fact. In a cost review it turns out to be an opinion with a decimal point, and the exposure is that every optimisation decision taken on it inherits the ambiguity.
Four definitions worth fixing first
- Cost per request — state what a request is. An HTTP call, a user-visible turn, or a logical task that involved eleven calls. These differ by an order of magnitude on agent workloads, and the agent case is exactly where the number gets quoted.
- Cost per outcome — state the denominator's owner. Cost per resolved issue, per merged change or per approved claim. The denominator has to be something a business system already counts, or it will be quietly redefined whenever the number looks bad.
- Blended rate — state the mix. An average input rate is a weighted average across models and cache states. Report the mix alongside it, because a blended rate with an unstated mix cannot be reproduced.
- Savings — state the baseline and the period. A saving measured against a pre-optimisation month that happened to include an incident is not a saving. Name the baseline window in the number itself.
Why this is a control, not a documentation chore
A written definition is the cheapest control in AI cost management because it makes disagreement cheap. When the denominator is written down, a team arguing about a 20% improvement is having a real conversation about the work. When it is not, they are having a conversation about which definition to use, and that conversation is where optimisation spend goes to die.
It also survives contact with a vendor negotiation. A cost-per-outcome number with a written denominator is a number you can take into a renewal and hold a rate card against. A blended rate with no mix is not.
Keep the definitions boring
Four or five metrics, each one line long, each with a named source system and a named owner. Resist adding a sixth. Every additional metric is a definition to keep true, and a definition nobody owns is how a dashboard becomes a wall of numbers that nobody defends in a review.
Bottom line
Write the denominator, the mix and the baseline window next to every cost metric you report. It is the difference between a number that can be audited and one that has to be believed, and it costs an afternoon.