great_cto reports cost numbers on the board. Those numbers MUST be auditable, because a wrong "7,638×" claim killed credibility (see docs/blog/cost-dashboard-rebuild.md). This skill defines the format.
The 4-line cost section
Every PLAN-.md and ARCH-.md cost section follows this exact template:
## Cost estimate
**LLM**: $<low>–<high> (<N> calls × $<per-call avg>)
**Human equiv**: $<low>–<high> (<hours> × $<rate>/h)
**Infra delta**: $<low>–<high>/month
**Time to ship**: <hours> agent-time, <hours> wall-clock
Methodology:
### Why this exact format?
The board's `getCostHistory()` parser anchors on **line-start** "LLM" and
"Human" labels. Mid-line references are ignored to prevent the
$240-trap regression. Stick to the template.
## How to estimate each line
### LLM cost
For each agent in the pipeline, estimate:
- **Prompt tokens** = (system prompt size) + (context the agent receives)
- **Completion tokens** = (typical output for that agent type)
Quick reference for Sonnet 4 ($3/M in, $15/M out):
| Agent | Typical prompt | Typical output | Per-call cost |
|---|---|---|---|
| architect | 14k | 1.5k | ~$0.06 |
| pm | 6k | 0.6k | ~$0.03 |
| senior-dev | 8k | 0.8k | ~$0.04 |
| qa-engineer | 11k | 0.5k | ~$0.04 |
| reviewer (avg) | 8-12k | 0.6k | ~$0.04 |
| security-officer | 12k | 1k | ~$0.05 |
| devops | 9k | 0.8k | ~$0.04 |
For Haiku ($0.80/M / $4/M), divide by ~4. For Opus 4 ($15/M / $75/M),
multiply by ~5.
Sum across the pipeline stages that actually fire (use `gatesFor()` and
`reviewersFor()` from archetypes.ts to know the count).
### Human equiv
The human cost to do the SAME work without agents. This is the "if I
hired a senior engineer, how long would this task take, at what rate?"
- Senior engineer: $120-180/hour (mid-market US/EU)
- Staff engineer / specialist: $200-300/hour
- Domain expert (security, compliance): $250-400/hour
Estimate hours conservatively. A "small feature" the LLM does in 15
minutes might take a human 2-4 hours (it's never just the typing).
### Infra delta
Only count what's NEW. If the feature adds a Redis instance, count
Redis. If it adds 10MB/month of S3 storage, that's noise — don't list.
### Time to ship
Two numbers — both useful:
- **Agent-time**: wall-clock of LLM calls (typically 5-30 min)
- **Wall-clock**: actual elapsed including human gates (typically hours
to days)
## Sanity check before writing
Before committing the section to the plan, verify:
ratio = human_equiv / llm_cost
If `ratio > 1000`, something is wrong. Common bugs:
| Bug | How to detect | Fix |
|---|---|---|
| Wrong unit ($ vs ¢) | LLM cost ends in /M tokens not $ | Convert: tokens / 1M × price |
| Counting savings not spend | "Human time saved" not "Human cost" | Use cost of doing it, not value of skipping |
| Mid-line label pollution | Plan has "$X LLM | $Y human" on one line | Use multi-line format from template |
| Forecast vs actual mixed | LLM forecast counts toward total_llm | Separate forecast section if needed |
## Cost gates
For AI archetypes (`mlops`, `ai-system`, `agent-product`), the pipeline
opens `gate:cost` after architect's forecast. CTO must approve the
projected monthly burn before senior-dev starts.
Use the GATE template:
```markdown
## Gate:cost forecast
| Production volume | Monthly LLM cost |
|---|---|
| 1K req/day | $X |
| 10K req/day | $Y |
| 100K req/day | $Z |
Recommended monthly cap: $<cap>
Triggers above cap: <what alerts fire, who gets paged>
Anti-patterns
❌ Round-number theatre. "$0.50 LLM | $7,500 human" — looks suspicious. Use realistic ranges: "$0.50–1.20 | $225–360".
❌ Single point estimates. Always provide a range. Single numbers hide uncertainty.
❌ No methodology line. Just numbers without rationale is unverifiable.
❌ Hand-waved infra. "Some hosting cost" is not a number. Either give $, or say "infra: no change."
Example — good
## Cost estimate
**LLM**: $0.75–1.85 (3 tasks × $0.25–0.62 per Sonnet call)
**Human equiv**: $225–300 (1.5–2h × $150/h, mid-market senior)
**Infra delta**: $0/month (uses existing Express + Postgres)
**Time to ship**: ~15min agent-time, ~3h wall-clock (1 human gate)
> Methodology: tasks sized by line-count estimate; per-call cost from
> historical Sonnet 4 averages on this archetype's plans.
Ratio = 300/1.85 = 162×. Plausible. Defensible.