We run an AI interviewer that talks to candidates over phone calls. It started on a managed voice platform at roughly 10¢ a minute. The same interviews now run on a custom LiveKit pipeline at about 2.5¢ a minute, all in. For reference, Retell's pricing calculator defaults to 11¢ a minute and LiveKit Cloud's own calculator to about 4.8¢.
The stack
- Telnyx SIP trunk into LiveKit SIP
- Silero VAD plus LiveKit's turn detector (v1-mini) running on CPU
- xAI streaming STT, Gemini 2.5 Flash-Lite, xAI TTS
- Self-hosted agent workers and Redis for session state
Where the money goes
Per minute of an inbound call, at published list prices, assuming 600 characters of agent speech and about 3,000 input / 175 output tokens a minute:
| Line item | Rate | Per minute |
|---|---|---|
| xAI TTS | $15 per 1M characters | $0.0090 |
| LiveKit SIP fee (Ship plan) | $0.004 per minute | $0.0040 |
| xAI STT (streaming) | $0.20 per hour | $0.0033 |
| Telnyx inbound local | $0.0032 per minute | $0.0032 |
| Gemini 2.5 Flash-Lite | $0.10 in / $0.40 out per 1M tokens | $0.0004 |
| Turn detection (local CPU) | none | $0.0000 |
| Workers, Redis, logs | self-hosted | ~$0.0051 |
| Total | ~$0.025 |
A few things I didn't expect:
TTS is the biggest line. Almost a cent a minute, billed by character. Shorter agent turns save more than any model swap would.
The LLM barely matters for cost. Four hundredths of a cent. Pick it for latency and quality, not price.
Rate cards only get you to 2¢. The last half cent is workers, Redis and logging. No vendor lists that.
When it isn't worth building
On per-minute cost alone, building wins easily. Count engineer time and it gets closer. With about $600 a month of fixed infra and two engineer-days a month of upkeep (at $150k a year, 260 working days), a custom pipeline only beats an 11¢ vendor above roughly 20,600 minutes a month. Below that, stay on a managed platform and spend the time on the product.
The full migration, with the numbers behind the 75% cut, is in our 2.5¢ voice AI case study. If you want to run your own volumes, there's a voice AI cost calculator on our site.
Rates are from the xAI, Google, Telnyx and LiveKit pricing pages. Originally published on Axionry Engineering.
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