4. The three cost layers

# 4. The three cost layers

| Layer | What | ~$/mo | Scales with |
|---|---|---|---|
| **Fixed per instance** | Fly machine + volume + (self-host) ngrok | $6–8 | # of instances (1 per managed tenant; rooms share) |
| **Marginal per seat** | Stripe fees (Telegram delivery is free) | ~$1.00 | # of paid seats |
| **Per-seat usage** | LLM + embeddings + images + crawl + TTS | $0.50–8 | how hard the seat is worked |

Infra amortizes across a room (one box, many seats); the usage layer is the only one that's genuinely
per-person-variable — and the only one a flat price misprices. **Token-based pricing is precisely a meter on
layer 3.**

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