# What is cost per customer?

> Cost per customer is model spend attributed to a paying account, not to one person inside it. Why seat growth breaks the per-user view, with math.

URL: https://getculpa.com/what-is-cost-per-customer
Last reviewed: 2026-08-01

## Answer
Cost per customer is model spend attributed to a paying account rather than to one person inside it. In B2B a single customer can hold dozens of users, so cost per user hides the number your contract actually has to cover. Culpa, a local-first LLM cost, margin, and forecast ledger, tags each call to an account and rolls seat activity up to the customer paying the invoice.

## Why this happens
A per-user figure and a per-customer figure answer different questions, and the gap between them is your expansion curve. Land-and-expand is the whole B2B motion: you sign four seats and grow to forty on the same annual contract. Every one of those seats generates model calls the contract already paid for. Per-user cost can sit perfectly flat through all of it, because each individual seat behaves the same way it always did. The account's economics still change completely.

## What this usually looks like
- Renewal quotes get built from a per-user cost that has barely moved in a year.
- Nobody can state what your largest account costs to serve without exporting something by hand.
- Seat expansion is celebrated in the revenue review and never appears in the cost review.
- An account's spend crosses its contract value and the first person to notice is in finance, at quarter end.

## Common mistakes
- Using cost per user as a stand-in for cost per customer. Why it hurts: The two diverge by exactly your seat growth, which is the metric your sales team is paid to increase. Do instead: Attribute at the account level and report seats alongside spend, so expansion shows up on both sides.
- Attributing spend by whoever holds the API key. Why it hurts: Shared service accounts collapse many customers into one bucket and the split can't be recovered later. Do instead: Pass an account identifier on every call at request time, separate from the credential that authorises it.
- Reviewing account cost annually, at renewal. Why it hurts: By renewal the seats have already grown and the spend has already landed, so the review is a postmortem. Do instead: Track cost against contract value monthly, and alert when an account crosses a share of its own revenue.

## Self-check
- Name your five largest accounts by revenue, then name your five largest by model spend. Compare the lists.
- For your biggest customer, divide monthly model spend by monthly contract value. Read that as a percentage.
- Count active seats per account this month against the same month last year. That ratio is your hidden multiplier.
- Check whether a call can be traced to an account without a join through a user table that may have been reassigned.
- Find any account where spend exceeds a third of what it pays, and put it on the renewal agenda now rather than later.

## One account, eighteen months, no price change (illustrative)
A $500 a month contract served on GPT-5.4 mini at real rates of $0.75 per million input and $4.50 output. Seat counts and per-seat token volumes below are modelled on a typical expansion curve.
  One active seat: 2M input, 300k output = (2 x $0.75) + (0.3 x $4.50) = $1.50 + $1.35 = $2.85
  At signup, 4 active seats = 4 x $2.85 = $11.40 a month against $500 of revenue
  Eighteen months later, 46 active seats = 46 x $2.85 = $131.10 against the same $500
  Cost per user never moved. It sat at $2.85 the entire time
  Gross margin on the account fell from 97.7% to 73.8% with nobody changing anything
The per-user number was correct on every single day and useless on all of them. The account got 11 times more expensive to serve and the only metric that showed it was the one nobody was computing.

## Cost figures
Every figure carries its confidence and its source. No figure on this site is provider-reported.
- $2.85 — modelled monthly model cost for one active seat on GPT-5.4 mini [calculated] Source: (2 x $0.75) + (0.3 x $4.50) per million, using real GPT-5.4 mini rates from the price book, effective 2026-07-02. Per-seat token volumes are modelled.
- $11.40 to $131.10 — modelled monthly cost of one account as it grew from 4 active seats to 46 on an unchanged contract [estimated] Source: Both endpoints from the teardown arithmetic at real GPT-5.4 mini rates. A range because the seat expansion curve is modelled.

## FAQ
Q: What's the difference between cost per customer and cost per user?
A: A customer is the account that pays you. A user is one person inside it. In B2B those differ by the seat count, which grows over the life of a contract, so a flat per-user cost can sit on top of an account that got ten times more expensive.

Q: How do I attribute model spend to a customer account?
A: Pass an account identifier on every outbound call at request time, kept separate from the credential that authorises the call. Shared service accounts otherwise collapse many customers into one bucket, and no later analysis can split them back apart.

Q: What share of contract value should model spend be?
A: There's no defensible figure to quote across products, because model intensity varies enormously. The useful test is your own trend plus a threshold you set deliberately, so an account crossing it triggers a conversation before renewal rather than after.

## Sources
- OpenAI API pricing: https://developers.openai.com/api/docs/pricing

Run the free Cost Leak Scan: https://app.getculpa.com/scan?source=pseo&slug=what-is-cost-per-customer&cluster=metric

Machine-readable index of every guide: https://getculpa.com/api/pages
Human-readable index of every guide: https://getculpa.com/guides
Site overview: https://app.getculpa.com/llms.txt

Privacy: Culpa runs on your infrastructure. Your prompts and responses never leave it. Culpa counts calls to run your plan, and it fails open, so if it ever breaks your app keeps running.